I think our chihuahua has sprained his knee severely.?

My 10 month old chihuahua has sprained his right back knee. We took him to the doctor 3 weeks ago and the doctor gave us some animal ibroprofen(sp) for pain and inflammation. He got better for a couple days then a week ago he started limpin again. This time on both back legs. I can tell he is hurtin. :( Both legs are kinda turnin in what does this mean? We are takin him to the vet at the end of this week but I want to know why his legs are bowin in. Poor puppy is hurting somethin terrible. If any of yall can tell me why his legs are bowin in please do. Thanks. -Jess