What Causes Petechial Rash?

Hi, My 10 year old daughter has a petechial rash. I noticed this 3 months ago, it just keeps fading and appearing elsewhere. Meningitis and low platelets have been ruled out. She has complained of intermittent knee pain, has headaches, gets tired easily and has intermittent diarrhea... I'm not sure if these are related.
I am wanting to know:
1) Does there always have to be a reason for a petechial rash?
2) I was told last time that it was probably a virus, would a virus generally last for over 3 months?