My 11 Year Old Daughter Suddenly Started Having Severe Pain

my 11 year old daughter suddenly started having severe pain in her upper abdomen on the right side.
she complains of pressure and says it feels like someone is stabbing her. I had her take a couple of tums in case it is just gas, but that has not helped. when I apply pressure on her stomach she says it hurts more on her right than on the left. she does not appear to have a fever. she was home from school with a sore throat and headache. we took her to have a strep test and that came back negative. I don't want to take her to the ER if this is nothing, but then again I don't want to not take her incase it is something serious. what other things should I be looking for to determine what the cause is. she says she is not constipation.