If you're not sure, go to the doctor. An injury such as a sprain can get worse without treatment. In the meantime, treat it with the "RICE" treatment--Rest, Ice, Compression, Elevation: stay off the affected foot as much as possible; put an ice pack on it for 15 minutes, followed by 45 minutes with no ice, and repeat 4-5 times daily; apply an elastic compression bandage such as an Ace bandage or ankle brace; and elevate the affected foot by lying down with the foot level with, or above, the level of your heart. Hope this helps!
A sprained ankle has symptoms, like, swelling and pain especially when weight is applied. It can also be experienced through restricted range of motion on the affected foot. You should see a doctor if you have pain and swelling, or if you suspect a sprain. The color also changes and it may turn red and a warm feeling is also felt. This is according to information from, http://www.mayoclinic.com/health/sprained-ankle/DS01014/DSECTION=symptoms.