I have had the same for thing for a year now, pain comes and goes but the pain isint the redness urethra tip
I have a similar problem, but only after having vaginal sex with my wife. Oral sex and masturbation never cause the problem, and it's not every time after sex, just some of the time. It first happened a few years back, we use a contraceptive gel as part of birth control. She had bought a "bullet" to insert, rather than squirting the gel in. I think the tip of my penis was hitting into the bullet, because afterward I was extremely sore in the same way.
I think I may be allergic to the gel, and it's causes a skin irritation on the sensitive tip. After sex, the tip will be a little red, but doesn't starting hurting until after I urinate. After urinating, If I look at the tip, it's defiantly swollen looking, with the skin from inside the urethra pushing out slightly ( I assume from being inflamed). By morning I feel better, with extremely slight discomfort for a day or so.
Only once was it bad the next day, and that was one night that I felt a little twing feeling in my penis, but kept going. Eventually I had to stop, tried to urinate, and almost fell over in the bathroom. My penis was so sensitive I had to lay down on my back and not move. The following morning it still hurt and I called out of work. For a few months following that I was in mild discomfort. Since then I've even changed my masturbation frequency and duration, and am much more aware of lubricants and contraceptives we use in sex.
Lot of information there, but I don't think it's an STD because we're both monogamous, I've only been with her, and she's been tested since we met. I think I'm having a skin reaction to the contraceptive, but it would be nice to hear a doctor agree.