I used to get this with my old TB mare when I first started stabling her at night. With her it definitely wasn't windgalls, just an accumulation of fluid from not moving around. It's a while ago now, but I think it wore off after a few days, certainly not more than a week, as she acclimatised to standing in.
Windgalls are normally a very localised swelling at the base of the tendon / top of fetlock at the back of the leg. This was a more general puffiness all around the fetlocks.
http://nfstallions.info/ http://forestuprising.org.uk/