You are correct that perioral dermatitis is primarily pimply, but tretament can disguise it. Steroids in particulr suppress the condition a little, and then when you stop--WHAM--and I think that's what's happening to you.
I would consult with the dermatoloist about an oral therapy--peraps something other than tetraycline, just to be sure.
Just don't be discouraged, though--I'm confident this will all go away.
Take care.
Dr. Rockoff