What Causes Painless Swelling Behind The Ankles?

I've started exercising a little more than usual (dance) and I've developed two painless but swollen areas just above and just below and slightly behind (in what should be the recessed area between the ankle and the Achilles tendon) my ankle on the outside of each foot (inner is fine.) I'm 35, 5'9", female, and 200 pounds. I'd stop if anything hurt, but as I said, it's painless, so I'm only slightly concerned. What is this?