I was given keflex around 3 months ago for hidradenitis. i had an allergic reaction and stopped taking it . i've had hives ever since. why is that?

Not Keflex: HIves is a particular kind of rash, and rarely lasts more than a few days. And any rash due to cephalexin (Keflex) allergy would not last more than 1-2 weeks after stopping the drug. So whatever the cause of your current rash, it probably isn't hives and cephalexin isn't the cause. If you're taking any other medications, you might be allergic to one of them. Discuss with your doctor.

Chronic urticaria: Triggered by drug allergic reaction isn't uncommon,I am not sure of the kind of allergic reaction you suffered when exposed to the keflex then,but developing hives later on is considered a non-immediate reaction to the drug, can it last that long? Yes it can, can also come and go with some time lapse.Unfortunately no specific test can diagnose it with certainty except careful history, c an allergist