My Morkie is limping on its hind leg. What could be the problem?

Two things come to my mind when I hear about rear legs giving way suddenly. I will address both separately. 1) Luxating patellas. This is a fancy name for loose knee caps. The groove the knee cap sits in is often shallow with smaller breed dogs, especially with the Yorkie or Maltese. The knee cap can slip in and out of place on its own quite readily in some dogs. It typically is not painful but can develop arthritis with time and this causes more irritation. What typically happens is the dog goes to step on the leg, the knee cap is not in place and they cannot place weight normally. Sometimes they do not place any weight on it, almost like a little skip, or they hold the leg up. The knee cap typically goes back into place on it's own or the dog kicks backwards and this puts it back into place. The arthritis can be managed with joint supplements or anti-inflammatories but typically the knee cap problem can only be fixed surgically and is only warranted in pets with severe trouble. 2) Bone injury -- without a known trauma, such as a fall or being hit by a car, a fracture is less likely. This causes the area to weaken and can suddenly become painful. It may not break but can be difficult to walk on. If this was the case, I would expect your dog to be painful if that area of his leg is touched. If you feel up and down his leg with pressure, does he seem painful? Do you feel any areas that are swollen?