I have a bubbly blistery rash on my lower back a bit like cold sores, very painful and itchy. what could this be?

Herpes simplex I: Cold sores, are contagious and usually occur outside the mouth, appearing as small red blisters on the lips, chin, cheeks, or in the nostrils. Also known as fever blisters, 95-percent of recurrent cold sore outbreaks are caused by type 1 herpes simplex virus, not the type 2 strain that can cause genital herpes.

Shingles? : If the rash is in an arc and limited to one side of your back, you could have shingles (herpes zoster) caused from the varicella-zoster virus which causes chicken pox. Get this rash checked out by your doctor quickly as they can give you anti-viral medication to decrease the severity of your rash.