Limping on and off

My cat Lola has been limping/unable to walk over the past few days from time to time. The vet examined her, unfortunately at a time when she was able to get about normally (she even jumped up onto the examination table!)

However, I've noticed that occasionally her hind leg will give away (particularly if she stretches the leg or, as on one occasion, walks backwards) and she'll fall on the floor crying. Could something (like a joint) be clicking in and out of place?