Alarming

I just had intercourse a while ago and I came home and I noticed a reddish inflamed bump on the opening of my urethra kinda looks like th color of a bruise 😰. It doesn't hurt when I am just sitting here and before I had sex I used the bathroom and haven't noticed any pain while urinating. Could this be because of sex? Is it a uti or an sti? I've had sex plenty of times with this one guy and nothing like this has ever happened. Please can you tell me what this might be? Thanks