Rash On Thigh, Hospitalized For Internal Bleeding. Taken Nexium. Could This Be Heat Rash?

I got this rash on the outer layer of both my thighs about 8 years ago, after I was hospitalised for internal bleeding (due to black colour tarred stool, but then it is not). Some Nexium was injected into me before the CT scan , and a day later, this rash developed. At first, I thought that it was some kind of drug allergy . I have been to the skin doctor for drug allergy testing, and the result was negative. However, the rash is always there, and is worse after exercising (coz of sweat) and also after shower etc. Can this be heat rash and is it permanent? I am a 36 year old female, with slim (average size body build), and healthy body fat level.