i have this shiny red rash on the underside of my penis, it does not itch or hurt. It looks like its rubbed raw but it isnt, the skin is thicker than the rest of the skin on the penis and has a rubbery feel. it showed up after i had unprotected sex with this girl but i waited a few months and went and got a std test "i figured that would explain it, and was 2 embaressed to ask about my penis" but it came back neg. so here it is 6 months later, i thought it would go away but it hasnt. its just kinda there, even if it isnt ne thing serious the ladies prob wont belive it. i kno i would want to mess with a girl that had something like this. i dont have insurance so i doc wont see me unless i go 2 the emergency room.... and thats a lil extreme.
You said you tested negative for STD's so apparently you don't have one... of the ones they tested for. Find out which ones they tested for, and do another bout of tests, this time for the ones they didn't get.
Aside from that, if it doesn't itch or burn, then I wouldn't honestly worry about it. Wear a condom until you know.