From what I can gather online, it seems that it's not a completely or well understood problem you have. I know that Mesacol is typically used for Ulceritive Colitis.
Hope the following info from online sources gives you a more complete picture of the condition and I wish you well in your recovery:
A rectal ulcer is an area of the rectal wall that is red (erythema) or has an open sore (ulceration). The condition is sometimes referred to as solitary rectal ulcer syndrome (SRUS), although the lesions are not necessarily solitary. Multiple isolated areas on the wall of the rectum may be affected. The rectal ulcer may cause bleeding and pain during the passage of stool. Rectal ulcers are rare, and there is a general lack of awareness of this condition.
A primary cause of rectal ulcers is rectal prolapse, a condition in which the lower end of the rectum protrudes through the anal orifice. Other causes may include constipation and straining during bowel movements, deficient blood flow (ischemia) to the rectum, direct trauma as a result of inserting a finger (rectal digitation) or other foreign object into the rectum, and anal intercourse.
Certain systemic diseases (oral ulceration, erythema nodosum, sacroiliitis) may also increase the risk of developing rectal ulcers.
Risk: Individuals with certain psychological disorders, such as obsessive-compulsive disorder, are at higher risk of developing rectal ulcers if they practice abnormal toileting behaviors.
Incidence and Prevalence: The incidence of rectal ulcer is estimated at 1 to 3 individuals per 100,000 per year (Felt-Bersma 7). Approximately 26% of individuals with a rectal ulcer are misdiagnosed. The condition affects men and women equally, and it can develop at any age. The majority of individuals with rectal ulcers are 50 years old or younger, with 25% of individuals over age 60 (Felt-Bersma 7).
History: Individuals with rectal ulcers will usually report rectal bleeding, which is the hallmark of this condition. Passing mucus, abdominal cramps, painful (but involuntary) straining while passing only small amounts of fecal material (tenesmus), constipation, diarrhea, and painful spasms of the anus (proctalgia fugax) may also be reported. The individual may also report a sense of anal obstruction. Pain is often localized to the region around the anus (perineum) or the lower back (sacral area) and is usually described as dull, continuous, and unrelieved or unchanged by defecation. About one-quarter of all individuals with rectal ulcer report no symptoms.
Physical exam: A gloved finger inserted into the anus (digital rectal exam) may reveal tenderness and bleeding. Also, a localized area of tissue firmness or hardness (induration) may be felt during the rectal exam.
Tests: The diagnosis of rectal ulcers can usually be made by examining the inner wall of the rectum using a flexible, fiberoptic viewing instrument (sigmoidoscopy). Open sores (ulcerations) will be seen on the rectal wall approximately 57% of the time; bulging, nonulcerated tissue (polypoids) are responsible for the problem 25% of the time, and inflammation (hyperemic mucosa) may be present in localized area(s) 18% of the time (Felt-Bersma 8). A tissue sample (biopsy) of the lesion for microscopic examination will be taken during a sigmoidoscopy to rule out cancerous conditions.
High-frequency sound waves (transrectal and endoanal ultrasonography) may also be used to visualize the rectum. Barium thickened to the consistency of a stool may be introduced into the rectum. Evacuation of the barium will then be monitored via fluoroscopy and videotaped (video defecography) to assess any abnormal muscle control in the lower bowel. Physiological studies (anal canal electro-sensory threshold, rectal distention threshold, resting anal manometry, anal squeeze pressure manometry) may also be performed.
Treatment will be either conservative or surgical. Conservative treatment may include local application of a drug (human fibrin sealant) to stimulate cell (fibroblast) and vessel (vascular) growth; taking stool softeners; assessing any relevant psychological factors; and encouraging the individual to stop using laxatives, suppositories, and enemas.
Surgical treatments may include stitching (suturing) the ulcerated areas closed or removing (local excision) the area with rectal ulcers. If rectal prolapse has occurred, the portion of the rectal mucosa that has prolapsed may be removed (prolapsectomy) or repaired (encirclement, abdominal, or perineal procedures).
Alternatively, the entire section of the rectum containing the ulcer may be removed (resection). For some individuals who are surgically treated, a temporary or permanent opening (stoma) may need to be surgically created to permit defecation (colostomy).
Prognosis - There is no specific cure for rectal ulcers. Symptoms may be improved by either conservative treatment or surgery. Success rates for conservative treatment using increased fiber in the diet vary widely from 20% to 70% (Feldman 2298), but it is uncommon for tissues to completely return to normal. A combination of taking stool softeners, increasing fiber intake, and discontinuing straining while defecating produced improvement in about two-thirds of individuals. In about half of those who showed improvement, the ulcer was completely healed (Feldman 2298). At times, patients may require psychological counseling with behavior modification to assist with changing bowel habits.
Surgical excision or suturing closed the ulcerated region, in combination with correcting rectal prolapse, produced symptomatic cure in about 50% to 60% of cases (Feldman 2298). However, even after surgery, rectal ulcers may recur.
Complications of rectal ulcer may include excessive rectal bleeding (hemorrhage), extreme disturbance of bowel function, formation of an abscess, formation of a hole (perforation) through the rectal wall beneath the ulcer, and surgical infection.