I have a really bad rash on my upper thigh, what can be causing my rash?

Intertrigo or contac. duration of the rash,size of rash,more description Need to see a doctor may apply vit E oil. Take Benadryl (diphenhydramine) 25-50 mg twice daily-be careful of the drowsiness.
Difficult to tell. There are many rashes that can happen on that specific area. It would be good if you have your provider take a look at it in order to tell you what it is.