Small dogs, especially small dogs that were bred by people who didn't bother to do health testing first, are predisposed to something called a luxating patella. In human terms it amounts to a trick knee.
There are varying grades of subluxation, from grade 1, which is mild and causes them to "bunny hop" when they run holding the affected leg up, to grade 4 which is painful enough for them to not want to put the paw down at all, even when they are just walking.
A grade 1 and even a grade 2, especially in a small, lightweight dog, can usually be left alone. A grade 3 and certainly a grade 4 should be surgically corrected so that the dog doesn't have to live in constant pain.
I hope this addresses your question. If you have any other questions, please post back.
Have you seen a vet if it is starting to be a bigger problem call your vet and ask about it .