What Causes Burning Sensation On Penis After Masturbation?

I have a question regarding my penis. I am 24 year old male, not sexually active and have not been since June. In October, just for fun, I got tested for every STD in the book and to no surprise came back negative. I had jock itch in my pubic area so, although I wasn't sexual active, my DR thought it would be good just ot rule out. It turned on fungal jock itch was correct. Since then occasionally, after masturbation, I get a intense bruning sensation on the underside of my penis that can last for 2-3 days. Its a small MM area and there is no inflamation, redness, bumps, or anything that is not normal. I figured if this was friction burn it would go away or be red. ANy idea?