I hope Mel doesnt have Luxating Patella, Peanut has it on both legs and it is very sad. Its a hereditary condition that sometimes needs to be operated on. Peanut has to go to the vet in like 2 weeks to see if he need the operation. I am currently giving Peanut Glucosamine pills in hopes that his legs gets better. You should ask your vet about the pills. They are cheap and are "supposed" to build cartilage and strenghten the bones on the knee and all that good stuff.
Until you go the the Vet, you should take it easy on Mel and not let her jump up and down on your bed and couch because it might make it worse if thats what she has.
There is also a good article posted on the Chihuahua Articles section of this forum on the second page about Luxating Patella, thanks to ilovesadie.
You should read that and have the vet look at Mel.