Penile Irritation? Possible Diagnosis?

Alright, so about 3 days ago me and my long time girlfriend had unprotected sex, as we have for a long time. No lube or anything, it was kind of random. A few hours afterward, I noticed that my penis was kind of itchy, like right at the tip of it, almost as if it had an itch right inside the opening. It doesn't bother me until I pull at myself like I'm trying to adjust myself, and when I urinate, it doesn't necessarily burn throughout, it just kind of has a sting at the opening. Could this be some sort of STD, a UTI, or could I just have an abrasion. Theres no leaks coming out, theres no visible damage, other than a tiny bit of reddening as any irritation would have. It also has a slight burn when I "cut off the stream." Even though I am not urinating when I "flex" or whatever you want to call it, it still has a bit of the burn. It doesn't always hurt when I touch it, as a matter of fact right after I urinate, if I adjust myself it doesn't bother me at all and feels fine, maybe because they skin isn't dried since us men can never get the last drop out. I've had a little pain in my lower back here and there, but it could be from work or anything, and today every now and again my testacles feel like I've been "bag tagged" but again could be from work. I do heavy lifting and such. What do you think this could be?