I developed a large blister on the back of my heel a month ago. it popped and is oozing a greenish fluid. it smells terrible. the area surrounding the blister is red and sore and warm. my ankle is swollen. is it infected?

These are characteristics of an infection and since the feet are farthest from your circulation, healing is often impaired. Also unavoidable walking daily hinders healing. For these reasons, I recommend consulting a podiatrist for expert treatment and best follow-up care for this infection.

Infected until proven otherwise. The sooner you have it assessed, the better as infection may progress fast. Would suggest starting with your doc or an urgent care center.