I used prednisone 40mg daily for a week for vasomotor rhinitis and sinusitis. I now have watet retention, swollen and warm feet and I picked up 4kg!

Side effects. Prednisone in that dose for 7 days can do all those things but if you are usually healthy these side effects should gradually wear off over the next week or 2 as long as you restrict extra salt and calories. A one time dose of a diuretic could expedite the fluid loss but take a potassium supplement with it.
Common! Water weight gain and real weight gain is common with the dose of steroids you took. Some should come off (water) fairly quickly, but if you put on some fat weight, the diet and exercise are key!