No STD causes flakiness or itching of the scrotum, or the sort of rash you describe. Every one of the hundreds of things that cause skin rash anywhere on the body can affect the genitals, and I don't know what's going on. But tinea cruris (jock itch) probably is the best bet. Hydrocortisone not only doesn't help tinea, it can make it worse. You might try an over-the-counter antifungal cream; ask the pharmacist for a specific suggestion. If that doesn't clear it up, see a health care provider for proper diagnosis. But this definitely isn't an STD.
Good luck-- HHH, MD