Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that your girl Cha Cha is unable to make it more than a few stumbling steps without collapsing.
I am glad to see that she has lost 2 pounds, that is a huge achievement in a dog that only weighed 18 pounds to start.
While arthritis can make it difficult for them to walk comfortably I would not expect her to collapse after a few steps, unless she has another underlying joint abnormality such as luxating patellas, (knee caps that float in and out of proper position), or intervertebral disc disease such that she cannot properly feel or place her legs and support herself.
Does she seem overwhelmingly painful? Or is it more that she is incredibly weak?
Metacam is a very good nonsteroidal anti-inflammatory. Has it seem to make any difference for her?
Does she seem winded and is she laboring to breathe? That would go along with primary heart or lung disease. I would check her gum and tongue color when she collapses. If her gum/tongue color are blue/gray or very pale then we need to look into heart or lung disease or anemia as a cause of or contributor to her collapse.
She is young to have such severe arthritis, so perhaps she has a tick borne disease. I highly recommend a blood tick panel to look for underlying diseases.
If her heart and lungs are normal, she is negative for tick borne diseases, and seems truly weak rather than painful then I highly recommend a referral to a veterinary neurologist. They can evaluate her for intervertebral disc disease or other neurologic diseases that are more likely to lead to weakness.
Please let me know if you have any further questions.