Chihuahua 8 mo old limping on hind leg today?

Chi Chi, my 8 month old female chihuahua, seems to have injured her hind leg. Last week she had a stumble after confronting the metal grate on the sidewalk during a walk and limped for the rest of the day. Then, it seemed to be doing better. Last night she jumped off the bed and cried in pain a couple of times. Her hind leg stuck out a little awkwardly and then she hobbled a bit but it seemed okay. I am unsure if this is related to the grate/walking incident or not because I'm having difficulty determining which leg is injured. I can touch each part of her leg/legs and she doesn't cry. However, she sits funny now and when she lays, she lifts her left hind leg almost behind her head when you get close and then she kind of nips at herself, then cleans her privates as usual. She wont jump on the couch or bed now and also seems very sad. Is this something to do with her patella and "my" fault. I feel just terrible, and scared for her.