This happened to my last cocker, Sophie.
The first time it happened she was 2 years old, we rushed her to the Vet and he xrayed her. He then told us that she had arthritic changes, told us she would have to be on medication for the rest of her life (it worked out a £8 a week, not a lot today but this was 16 years ago) and that she must have virtually no exercise again, ever.
We came home and I broke my heart. Me and OH sat at the kitchen table with these tablets on the table between us and looked at Sophie, who by now was showing no sign whatsoever of there being anything wrong and was bounding round the house. OH picked the tablets up and put them down the toilet saying that she was not having them and if she got arthritis in later life, we would deal with it then. She didn't have one of the tablets.
The problem did happen again, we worked out it was when she was running and she swerved quickly. What we did when it happened was we carried her home, if near enough (she was only tiny), OH sat her on his knee and massaged her shoulder for about an hour, then we took her a walk up the road. She started off on 3 legs and by the time we got to the top of the road (8 houses) she was walking perfectly on 4 legs.
She lived until she was 14 years of age and was still walking up to 7 miles at a time until about 3 weeks before she died, with no trace of arthritis.
By the way, when I next went back to the vets I was really worried what they would say (not as stroppy as I am now!). I saw a different vet, he looked at the notes, looked at Sophie, back at the notes, back at Sophie, and never mentioned the problem. Nor did any of the other vets we saw, including the one who diagnosed her. If it was now I would want answers, I have always said it was a trial of a drug. You may wonder why I kept going to the same practice, but to be honest its a brilliant practice, the vet in question was the one who had to put Sophie to sleep and he was absolutely wonderful with both her and us. I still go there with Ellie and, strange though it may sound, really trust them, although I would not be afraid to speak up now.
Anyway, I really hope that Tilly's problem is just like Sophie's. As Sophie got older and stopped dashing about so much, the problem stopped happening.