Holly my maltese cross is 4 in December of this year. About 6 weeks ago I came home to notice her favouring her left back leg - I was only out for 20 minutes, so thought she must have jumped akwardly or something. She walked on it normally mostly and by the end of the week, I hadn't noticed any favouring at all. Over the past 6 weeks, I've noted a little bit of occasional limping - maybe once a week, twice at the most, still wanting to chase her ball and play and go walking.
She's always had clicky joints and lies like all Malteses that I know with her legs straight out at the back. Today however, she seems to be favouring the leg alot - not putting weight on it while eating her dinner (no appetite loss at all) , when I picked her up, and felt the knee, I felt a kind of pop.

She will be going to the vet next week for vaccinations, and I will get it checked, but a google search has me a bit scared - could it be luxating patella or are there other things that cause this?

Is glucosomene worth using?

If we had to go the surgery path, worse case, how expensive is this going to be?

Thankyou!!