There are several reason this can happen. You could be retaining fluid by eating/drinking high sodium foods. Sitting/standing for long periods of time. Certain medications can cause this too. If this is something that doesn't go down, you need to see your doctor. You may need a water pill to help with this. In the mean time lower your salt intake, don't drink soda pop, drink more water which will actually flush your system as well. Coffee is a natural diuretic as well. Elevate your feet as often as possible. Apply a cool towel to the feet/ankle as warm/hot weather does cause blood vessels to dilate. If severe enough you can also purchase circulation stockings at your local pharmacy, as this will compress the fluid back into the veins. So do try that for the day, if no change then do follow up with your doctor tomorrow for a proper in person evaluation.
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