Why does this happen to me?

i could eat the littlest of things and get a stomach ache (pain in stomach need to go to the bathroom ect.) weather is like 5meat (little ones) with a cookie. or actual dinner or breakfast or lunch.. i always seem to get a stomach ache so i have had tumss to bring to skool with me and this stomach thing has been going on for like 3 weeks now. i need to take tums before i go to skool and sometimes after lunch and i cant go to the bathrom all the time at my skool because first off i am trying to learn. second the teacher is trying to teach me. n third i can only go to the bathroom ones so having a stomach ache just doesnt cooperate with me so i use tums or pepto. can someone tell me what is wrong with me? i have a stomach ache again i was at a birthday part i had (a cupcake. a few chips. a paice of fuge and 7 meat that were small) didnt eat lunch so i was kinda hungry. and now i have a bad stomach ache.. is there something wrng with me? ive had a stomach ache litteraly for like 3 weeks an i am kinda becoming sick of it. i also have a charity thing to go t soon and i dont wnna stomach ache so i will be taking some tums..idk if that helps but.. whats wrong with me): im 16