How Can Swelling Of Ankles Be Treated?

Yesterday, for the second time in two weeks, I woke up and my left foot and ankle were swollen. I am 23 and haven't had any changes to diet, exercise, etc. I have been stressed, so am wondering if it is something to worry about. There is no pain, but feels like slight water retention, I think. It goes away after a day or so, but I am nervous. Is this something serious? What could cause it? Allergies?