Long before I got to the diagnosis given by your doctor, I had come to the conclusion that you had tinea. Tinea is the medical term used to describe a fungal infection. Fungal infections such as this as almost never the result of sexual contact but rather are a consequence of the moisture present (between your scrotum and thigh for instance) providing the opportunity for fungus to grow. Other terms for tinea are "jock itch" in men, "diaper rash" in babies. It can happen to anyone and is not an STD. Fungal treatment is just about always effective.
Your results are were negative before and there is no reason to think that they might have changed. You do not need further testing. Time to move forward.
Hope this helps. EWH