My Husband Was Prescribed Bactrim DS, 1 Tab Bid, For

My husband was prescribed Bactrim DS, 1 tab bid, for cellulitis 3 days ago. The next day after stating the meds it began draining. The redness has shrunk, but the center looks like a blister that needs to be popped which is slowly draining. We were in the process of changing his dressing and as he was removing the old one the skin of the blister came off and blood started to drip out. Is this part of the draining process or something worse?