I had protected sex with woman with hsv2 5 week ago. i have lower back pain and itch on butt.been feeling it 4 a few weeks. is this a sign of herpes?

Low risk, no worries: Herpes doesn't cause low back pain or "itch on butt". The chance of HSV2 from one episode of unprotected vaginal sex with infected partner is around 1 in a thousand. With condom protection, it's still lower. Don't worry about this. Also look at it this way: If you've had sex with say 20 different women, you can assume 3-10 of them had HSV2. The one partner you knew about isn't any more risky.