What Causes Red Rash Around Penile Head?

Recently, I've had an issue with a red rash around the head of my penis. it doesn't hurt when I urinate but it does feel irritating when it brushes up against my boxers - just kind of sensitive I guess. Also, just to be sure, I went and was tested for gonorrhea, syphillis and chlamidya and they all came out negative. What could this be? Any recommendations on meds to try?