Is this an internal door or external? I have known plenty of external doors to swell due to damp over the winter - but not an internal one.
If internal, which room is it in? Kitchen or bathroom, or somewhere that does get damp? If so, I'd want to look at reducing the damp in that room first - via extractors etc - since it is likely to cause other problems.
Anyway, if it is a question of damp, then the usual solution is to take it down and plane a little off in my experience - but be aware that you will then have a larger gap around it when it dries out again, so some kind of draught excluder may be a good idea.
However, the usual thing with internal doors is that one of the hinges - normally the top one - has become loose or bent. Tightening that up, or flattening it out, would be the solution there. You might need to look at the screw holes if it is loose - put a matchstick in if they have become loose - or use longer screws.