What would cause your leg to swell at night when asleep?

for the last couple of weeks my Left leg was swollen so bad I could hardly bend it.My knee has been giving me some pain but I just shrugged it off hoping it will go away.But the leg stays swollen until I can get up & move around.In May 2001 I had a triple byp & the leg leg artery was remove for the surgery--could this be causing the swelling.I don't think it would have waited this many years if it was.Or could the knee have something to do with this?I'm 65 & work as a janitor & it seems to do okay as long as I'm up moving around.Any serous suggestion would be appreciated.Tnx