May be the weather or temperature. A wooden door can expand in a warm / humid environment. In many cases, the 2 or 3 millimeters clearance on each side of the door can easily fill in through wood expansion. This can also be a result of the door coming in to direct contact with water. You'll need to take a small amount of wood off of the door's edge. This can be done using a plane or belt sander. Also check the hinges to make sure they're screwed tightly into the door frame. (Actually, you should do that first.)