Don't panic, but you need to get to your MD and have your heart checked. Why I say this is b/c I have a heart condition (a damaged left vent.)and often have the same symptom in my left ankle/foot. It is called "endema" (sp?) and is caused by the heart not have enough "oooph" any more to pull up the same amount of blood from the foot as went down. There's really not a whole lot that you can do. Keeping your leg(s) elevated, esp. above your heart e.g. lay on the sofa with your foot on a pillow,plus wearing a "granny sock" (compression legging) will help. The MD will probably give you "pee-pee pills" to avoid water build up. I'm not saying you do have a problem with your heart, only that you might..and you need to get it checked out. B/c, I do and I have the exact symptoms.
(And, yes you can also have this conditon on the right foot/leg if for some reason or other the reight side of your ticker isn't in A-#1 shape.)