What Causes Discoloration At The Back Of Leg?

I have a large discolored area on the back of my leg. Looks like a bruise but it s not. It started out as a small area last summer. It was itchy, but looked like a bruise. It hung around for a while, despite attempts to use topical treatments on it. It seemed to abate for a while, but it s now larger and another similar spot has appeared.