I'm 3 weeks 2 days out and having do e kind of reaction breaking out in hives... Only on my back? Anyways... Is it safe to take Benadryl ?

According to this sleeve manual it is safe:

http://www.peachtreebariatrics.com/docs/Gastric-Sleeve-Patient-Manual.pdf

However, I think it's really better to get medical advice from your own doctor/health care provider than from strangers on the internet. I know your doctor probably isn't available right now. If you have insurance, there may be a 24 hour nurse hotline you can call. They may also be able to advise you as to whether this is really hives or something else entirely. Good luck!