What Causes Sudden Dizziness And Nausea With Headaches?

My 18 year old daughter has experienced sudden dizziness and Nausea, that turns to shakes and a headache. She is 5'4 weighs @125-130 lbs. This just happened after she left a hard cheer practice, and it has happened in the past as well, generally after a work out of some sort! I am looking for possable answers to help her out!