Am I really sick, or is it in my head?

I have been sick for awhile now and I usually get sick right before bed. I have had diarrhea for about 2 months so my mom took me to a gastroenterologist. She did a colonoscopy and said my small intestine is mildly inflamed, but the don't know why. Every time I tell my mom I don't feel good she says "You always don't feel good before bed, Its all in your head!" I sometimes believe her and hate myself for "faking" it, but I don't know if I am. My mom is the best mom ever, and just wants me to go to school, and one night I was crying because I didn't feel good, and she said she was sorry for saying it was all in my head, and apologized, but the next night she was back to saying it was all in my head. Please answer and help me!! Thank you so much!!