What Causes Red Rashes On The Inner Thighs?

I have a HUGE red rash on my inner thighs. It itches REALLY bad. Its starting to spread around my right thigh and has now appeared on my left wrist.
It is VERY uncomfortable and looks horrific. Please help soon. I have had this for about 6 days now. I don't think its a heat rash since its now on my wrist. I am a 16 year old slim female. I am a vegan and eat really healthy. I have had Mono for sometime now and had Parvo about a month ago but I know this can't be the Parvo again since It looks and feels completely different. I feel fine except I keep getting bad headaches and occasional neck pains. PLEASE HELP!