Yeah, this is a tricky thing. It can happen once in an extreme situation in any dog, say jumping for a frisby and landing a little off. Small dogs are much more prone to it and when it starts happening in normal excersise situations that is the time to worry about it. If it happens during walking, trotting or normal running more than once then you have to get vet to advise. I really believe it is a condition that has been vastly overlooked - like you say, you can pop back yourself - and I think your vet deserves a big thank you for giving you a breeding warning. It has been overlooked as a minor thing for far too long!
We have the video clip shows on TV, I'm sure the whole world does, and a favourite is the dog that pees as it stands on it's front legs....seen it? These dogs have severe patella luxation and are not being funny..sorry...just makes me mad!!!

It's a serious problem in small breeds, as if they didn't have enough, but has been sadly overlooked. Now, in this country, at least, correction is seen as common surgery - I think this is really, really sad as there is no real need for it.

Pam,I'm so glad your dog is not too badly affected and well done you for spotting the signs of something wrong.
Donnab, please let us know what the vet says - there are lots of other things it could be, so don't let me get you down. Even if it is patella luxation that does not mean the end of the world, Honest. Anyway, let us know!