What Causes The Outbreak Of Itchy Blisters?

I have a skin irritation. It starts out with a small blister, no puss, just clear. Once it drains, a scab will appear. After the scab falls off, it leaves an indention in my skin. The skin itches, but there is no pain. I have been taking Bactrim & Doxycicline. Can you tell me what I might have.