If it is hives and they are making her itchy then the possiblity of her rubing until they are open wounds is possible. A little bute twice a day with a shot of steriods, and maybe some Hydroxyzine from you vet would benefit her. Keeping her in the stall is good but I would strip the stall and put clean and/or different bedding in. The silver sulfadiazine cream is good for those types of wounds. Also it is burn cream so great to keep around the barn/house for horses/humans. IT is a very soothing cream. You may want to get a little betadine/povidine scrub (soap) and wash the wounds onces a day, this is a surgical disinfectant, If no feed store is open you can sometimes get it a walgreens. THe scrub would be best but if they dont have it the solution will work. I would wash the sores once a day gently and then apply you Silver Sulfa cream.

In general the reaction seen when a horse gets penicillin in the bloodstream is very noticalbe they become very overly sensitive, the easiest way to describe it is they freak out. I have been around two horse that when given a shot of penecillin the needle must have hit a small blood vessel on the way out and the horse began to freak out immediatlely. I have never seen one get hives though.

IF she is still getting more of them I would call you vet on Monday (if you cant get someone out today) and have him recheck her maybe draw some blood and run a panel, and maybe see if it would be a good idea to culture swab a couple of them and send them off to the lab and see if they show anything.

Hopefully it is just as simple as hives and some drugs will do the trick. Let us know what happens, good luck.