Doggy dragging butt.?

I have a 17 month APBT. The last couple weeks I noticed her dragging her bum across the floor. I checked her poo for worms and there were none. I chalked it up to an itchy bum. Recently I found light streaks of blood on my bed sheets where she is fond of laying at night. My husband and I looked at her more closely and there are little red bumps along the inside of her tail. Not so much on her butt yet her butt hole is reddish and looks irritated. She has these bumps other places. On her mouth and tummy. Last time the vet prescribed a anti allergy medication and she was fine until about last month when the bumps on her mouth came back. I am worried about my girl, does anybody have any ideas as to what may be causing my baby so much irritation? I will get her back to the vet as soon as I get paid but that still a couple of weeks. Any ideas what the heck this is?