Why do My cats hips wobble?

She sometimes loses her balance and falls a bit. She is not limping and the dr. looked her over and didnt see signs of artheritis. This has been going on for several months and now my time at home has gone from enjoying being with her to worrying about her. She can still hop onto chairs and such fine. Also as a note, she used to bury her waste in the litter box, but not she keeps it uncovered and sometimes accidently shoots over the edge. She used to squat and no longer does, but does lay down to eat and drink. I clean the litter once a day, though i have missed a day here or there. I dont know what to do... primary concern is about the wobbling though. It started after we moved.