Hi there!
Collagenous gastritis is a very rare cause of GI related issues and may occur in association with other inflammatory/ autoimmune conditions. Symptoms may be varied, depending on the type and severity of the condition and may include abdominal pain, motility/ related issues, nausea/ vomiting, nutrient deficiencies etc. The condition is studied and described in two clinical forms. The first (the rarer among the two), occurs in children and young adults presenting with severe anaemia, with superficial involvement limited to the mucosa and a nodular pattern. The second occurs in adults with symptoms of chronic inflammation and motility issues. Management is similar in both cases consisting of anti-inflammatory/ immune suppressive therapy and prognosis is variable. Regular monitoring of the progress of the condition may be required in most cases. I would suggest discussing the situation and the suggested management plan in detail with the treating gastroenterologist.
Hope this is helpful.
Take care!