If it was swollen and dry immediately after sex, then it must be what it sounds like anyway--irritation from friction. No STD comes right after sex. The masturbation probably contributed. When swollen skin goes down, the skin can look dryish, just as you describe. Just moisturize and avoid sex for a couple fo days and you'll be fine. If the symptoms persist, please see a skin doctor.
Best.
Dr. Rockoff