Our previous dog, Not a Hav, had a lax petella(s). She too could put both of them back into socket on her own, which was good, and her with small size - she was 12 lbs- the Dr. said that odds are if she could put it back into socket on her own, she probably would do fine without surgery. Compared to a big dog and all the weight that would bare on it daily. He recommened a joint supplement called Cosequin, which I used up until she passed away last summer to help ease the ill effects and keep her ligiment and such from what I perceived as 'wearing' away. It was a tablet form I broken in half. She ate it by itself with no problems.
BUt yes, have the vet check her pads, knee area for any injuries, of course! My friend had a dog with a splinter that sort of reacted like you described.