Does herpes cause a constant slight burn in your back,groin and legs? symptoms for last year since random sexual encounter. could it be in my head???

Not herpes: Herpes does not cause such symptoms. There are many non-professional websites where people claim symptoms like this are due to herpes; pay no attention to them. "Could it be in [your] head?" No distant forum expert can say; you're the best judge. But such symptoms often have a psychological origin; and whenever someone suspects his or her own symptoms are psychological, usually s/he is right.