You may have a chafing rash which comes from the fraction of skin rubbing against skin, or perhaps there might be an element of fungus (" jockey itch"). Your first approach might be to apply one percent hydrocortisone every morning and terbinafine cream at night (both are over-the-counter), along with regular application of a moisturizer such as ordinary hand cream. Try your best not to scratch. If the medicated creams cause burning a little bit at first, persevere for two weeks anyway.
If that doesn't clear you up, please see a doctor. You have nothing to be embarrassed about!
I have been to the Doctor for this same fungus "jock itch". this is the first time I had it start behind the knee, almost clear up then start in the groin. The doctor put me on an oral medication plus the over the counter clotrimazole cream. My problem now is that the blistering and subsequent drain of fluid has rendered the scrotum and groin area to raw flesh. The pain is excruiciating! What can I do? I can't walk or allow clothing to touch it. The whole area looks like a third digree burm. What can I do and fast?
Soak in a warm tub. Apply nothing more than bland moisturizer. See your doc.