What Causes Swelling In Ankles?

I'm a 48 year old female I would say pretty healthy. I woke up this morning and my right ankle is swollen but it is only on one side dosen't really hurt. I have had no injury to it. Haven't been doing anything out of the ordinary. I do have high blood pressure and I take Lisinopril but have been on medications for many years. What could cause this? I have never had anything like this before.