Could be a few reasons

If you are on them all day sometimes a rather harmless condition called Edema sets in. Often soaking them in warm water/Epsom's Salts or just elevating them for a bit fixes it up

But Edema could also be caused by vascular issues/heart troubles

OR

If Diabetes related it could be early signs of Peripheral Neuropathy - nerve damage caused by vascular damage (small vessels in the feet) caused by sustained high blood sugar (and just a little high - any time your BG is higher that 7.8 mmols/140 points you are doing "some" vascular damage and it often shows up "first" in the feet being the (furthest region) of your vascular system

So your question should really be asked of your Doctor

Could also be a kidney problem,as Graham says it's doctor time