What Causes Itchy Red Rash Consisting Of Blotches On Pubic Area?

hello I have a rash on the pubic area just below my waist about an inch above my penis. sometimes it is itchy and the rash consists of small red blotches that go across my groin area. I had protected sex 5 days ago with a girl I didn t know. I want to know if this is possibly an std if so what is it and how soon should I seek medical attention.