Sounds like the same thing that I wrote about two posts earlier. My rash isn't itchy or tender, however, but it is exactly as you described in the same time period after sex. Perhaps an allergic reaction to the type of lubricant in the condom?
It just sounds like irritation, with residual redness that will go away if you leave it alone, not an STD. I'd ignore it.
Best.
Dr. Rockoff
Could be a yeast infection. It's not an STD