POOR little girl - it
sounds like LP (Luxating patella) ...but...She may have pulled a muscle or twisted her leg........
I AM NO EXPERT but have read many stories about LP - There's varying degrees of LP - and you NEED to speak with your vet and have her looked at. It's pretty common in small breeds....
This link speaks plain simple terms about the degrees of LP in a dog - there are many sites you could look at if you think she may have it -
http://www.vetinfo.com/dpatella.html...llas%20grading
I hope she feels better...Good luck