Stomach problems, Lactose Intolerant?

About two weeks ago I ate a banana and had a really bad stomach ache, in the center right below my rib cage, not knowing it was the banana that caused it, I ate another the next day and the same thing happened. I stopped eating bananas but it seemed like every time I ate I had really bad stomach cramps and pains. I started to think I had an ulcer, but very unlikely. I began eliminating foods from my diet so I started with dairy. Since I stopped eating dairy I have not had anymore stomach problems. Recently I ate a taco and forgot to take the cheese off and just that little bit of cheese caused my stomach to go crazy and experienced a lot of pain for a few hours. I may be lactose intolerant but why did this happen so suddenly? I have always been able to eat and drink dairy.