Tip of penis 2/3 red, no new sexual partners and no irritation

About 10 days ago, I woke up to notice the tip of my penis was approximately 2/3 red. There's no real irritation to speak of, and it seems to get slightly lighter and darker in color day by day. It was also dry until I applied cortisone and clotramazole. I have not had any new sexual partners, and my girlfriend and I are monogamous. I was tested for STD's at the beginning of our relationship and everything was negative. Does anyone have any idea what this could be?

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