Ankle swelling is a very common symptom. While it can sometimes be a sign of a minor condition, it can also be a sign of very severe problems. I would recommend that you have this evaluated by your primary care doctor
The most common cause of swelling of the ankles is simply "bad veins" or venous insuffiency. The veins are the blood vessels that return blood from the body to the heart. In the legs, because it has to fight gravity, these can sometimes become weak and so the fluid builds up in the legs. This can be managed easily with either a pill and rarely surgery.
That being said, there are some other major causes that must first be ruled out. A weakened heart can cause fluid to build up in the legs. This should be evaluated. Also damage to either the kidneys or the liver can cause this problem. These three conditions (problems with the heart, liver or kidneys) can all cause swelling and can all be very dangerous if not quickly identified and treated. Because of this, your symptoms require evaluation.
If there is a difference between the two legs, we sometimes worry
about clots in the veins. That being said, if both legs are similarly swollen -- this is less likely.
Talk to your doctor. Good luck!