My horse always cocks her hind legs?

my 6yo ottb has arthritis in her hocks & gets hock injections. when she is in her paddock or i am grooming her or shes in her stall, etc she allllwayss cocks her hind legs. its not just one she switches between them. if shea doing this all the time does it mean shes just always relaxed or could her hocks be bithering her again? she seems sound under saddle