In my experience, sprained ankles take forever to heal. But if you are worried about it, it doesn't hurt to get it checked by a doctor or a physiotherapist. I would try for the pt first I think. Going to the ER won't get you much besides an exray and told to come back in a week, then a new doctor will tell you to come back in a week, then a new one and a new one until finally somebody actually looks at your foot and your file and has a brainwave and does some proper investigating. Or maybe that was just me... But I think, if the swelling went down on its own and the bruising is gone, then a pt might help with ideas for how to let the ligaments heal properly.