What's wrong with my dog?

my beagle is 3 1/2 years old, and she just suddenly started having trouble walking because of her back legs and/or hips. she has never had this problem before, and when she tries to walk her wobbles back and forth uncontrollably and she usually falls or just leans on something because she can't stand/walk anymore. she also has had a fever. we plan to take her to the vet first thing tomorrow, but i would like to know what to expect...