I am not aware of stomach polyps commonly leading to blood in the stool and constipation. Normally, they would have to be biopsied during the upper endoscopy and then removed. Antibiotics typically do not treat stomach polyps.
It is more likely that the blood in the stool is due to the proctitis. Depending on the severity, steroids and antibiotics can be considered to treat the acute episode.
One consideration would be immunomodulator therapy to manage the chronic disease.
There are many other causes that can lead to constipation, and I would check the thyroid level as well as more specialized tests like an endoscopic ultrasound or anorectal manometry if the diagnosis remains non-revealing.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.
Kevin Pho, M.D.
KevinMD.com
Twitter.com/kevinmd