I don't think you contracted an STD. This sounds like recurrent irritation. Perhaps you're examining yourself more closely (too closely?) because of your concern that you might have picked up something. I'd leave the penis alone, apply 1% hydrocortisone twice a day for 5 days, use a bland moisturizer like vaseline, and show your doctor so you can be sure and then relax.
Best.
Dr. Rockoff