Thanks for posting your query at DoctorSpring.com
I have gone through your question in detail and I can understand what you are going through.
Benadryl does have some antianxiety property but its not a first line drug for anxiety. The abdominal side effects that you are experiencing are probably due to the acidity. The side effects are associated with tge use of benadryl.You will have to firstly stop the tablet benadryl. The symptoms may reduce in next 24 hours. You may drink plenty of water during this period and take some OTC antiacid preparations. If the symptoms do not receed in 48 hours time then you should see a doctor in that case.
Hope this helps.
If you have any further query do let me know.
Dr. Srikanth Reddy MD