Non-itchy Red Rash On Penis Shaft, What Is It?!?

Me and my girlfriend were both tested for std's before we started having unprotected sex and were both negative. But about 4 months after the test I started getting this reddidh rash on my shaft, its not itchy and there's no bumps. And usually it just looks kind of dry and flaky. But after sex it gets really irratated and the skin has even cracked and scabbed a little bit makin it sore after intercourse. Me and my girlfriend do tend to have rough sex. We have lubrication but sometimes we forget. Its not on the head of my penis just the shaft and tends to go away if I start putting 1% cortisone on it. Could this rash be from having rough sex, or should I be worried about std's?