What do I do if my stomach hurts alot?

well today in school they served nachos and a lot of older kids didnt get nachos. but i did. and everyone says to never eat the nachos becuase the chips and the cheese are expired. and i have never had my period so its not that. and my stomach hurts like alot. and it won't stop. and my parents tried to get me to take tylenol but it said children 12 and under can get liver damage....... please help me!!!!!! and i cant get the taste out of my mouth, it taste like i ate 5 bags of peanuts!!!!