What Could Cause Swelling Of Feet?

My feet have been swollen for a month now, i had an ECG, scan both legs for any blood clots and blood tests for the kidney. all were fine. the gp gave me some diuretics but it doesn't seem to do anything apart from increasing the number of time going to toilet. For the past year I have been on prednisolon for polymialgia. what should I do? many thanks,julia