Something very similar happened to me about a year ago while on Phentermine (which i had taken many times
before without breaking out in these hives).
After doing several rounds of steroid medication
and having the hives return, I was given an antibiotic to treat an unrelated problem. The hives cleared up almost immediately after taking the antibiotic. I can't say if the antibiotic cleared them up, or it was a coincidence but I was happy to see them gone. I hope this helps.