Hi, welcome to the forum, anti histamines is known to cause certain abdominal side effects like nausea, dry mouth, diarrhea, abdominal pain
, appetite increase and weight gain. Abdominal pain
is seen in around 2-4 % patients but the cause of such pain is not clear. In rare cases it is known to cause stenosing peptic ulcer and pyloroduodenal obstruction which can present as severe abdominal pain. You may have to undergo endoscopy to rule out peptic ulcer, gastritis etc. Take care and regards.