My 15 Year Old Daughter Has Had Diarrhea Since Sunday

My 15 year old daughter has had diarrhea since Sunday morning. She is eating somewhat today finally. She has not gone to school yet this week as her stomach hurts to even stand. She doesn't have a fever and she is not vomiting. Last night she started developing a rash, a small one on her tummy. This afternoon, the rash is wherever she itches. They are huge with raised welts. They look like huge mosquito bites that cover her entire back or leg, or stomach. ER Doctor was not concerned with half-dollar rash last night. He also said her tummy was soft, she had no fever and said she was mildly dehydrated. He ordered no tests. Are her two symptoms related? Any idea what she is suffering from. Her tummy hurts so much and her body looks awful and she is itching so bad....even with liquid Benadryl and topical Benadryl.