Other reasons could be from eating a poor diet high in salt and carbohydrates causing retention of sodium, laxative and diuretics abuse, or Neuromuscular disorders to name a few. When you experience swelling, try lying down elevating your feet above your heart level by 10". Also, drink more water, don't use laxatives, exercise daily, see if improper footwear is leading to this. Lastly, go see your doctor. If taking any meds and supplements be sure to discuss them with him/her.
Swelling feet in a diabetic is often connected with kidney failure.
There is a possibility of a cardiac etiology. Congestive heart failure also can cause such symptoms. But let's not assume a "worst case" scenario.
The inability to excrete fluids causes a built-up.
You need to see a physician for an evaluation. Diuretics are commonly prescribed.
Bring this to your doctor's attention at the next check-up.