My 16 Year Old Daughter With Down Syndrome Woke Up

My 16 year old daughter with Down Syndrome woke up to go to school. While sitting on the toilet she fainted. She ate well the night before and was well hydrated. After she "came too" and I was convinced she was O.K. I left the room to call the school to keep her at home to observe her. As I returned to the bathroom whe was attempting to stand and again fainted. He arms were slightly shaking but I did not think it was a seizure. I took her to the E.R. they ran blood tests, urine test and EKG. She looked uncomfortable and winced a lot. They said all the tests were normal and they gave her tylenol and sent her home. Should I obtain further testing? This has never happened before. She does not have a heart defect.