What Causes Redness And Burning Sensation On Penis Tip?

I've been having this problem for about a week now. The tip of my penis is red and it burns when I urinate. I also have strep throat. I've been with the same girl for over 3 years now and we've both been completely faithful so I don't see how it could be an std. I just don't know what could cause this. I am 22 years old 170 lbs and 5' 11". This is the first time I've had this problem