Hi, ive been having diarrhea each time i eat, ive had lots of test but each time they come up normal. Any idea?

ive had a tube down my throut an up the but it came bk normal, i get pains in my lower stomach/bowel, i have low b12 levels an ibs but its now each meal i need to go. Do u think there could be an underlining problen which these tests havent shown x