A little hitch in Liz's step, ughgh!

Lizzie went for her herding lesson, this was the last one with the current instructor and I have another one with an instructor that I really like next week. So last time Liz was herding I had thought I saw a really different gait then I was used to on Liz, and I had asked the instructor, who said well, some dogs get a little hitch in their back...but it did not look like a back problem to me. I did notice her back a little more roached then it usually is and a lot more hoppy type of gait and her "messing" with her legs, I do not know how better to describe it. I have not had a lot of experience herding so I did wonder if she was a dog that just had a hard time changing to a different lead when we made her go the other direction, humm, could that have been what is going on? I decided to just watch carefully and try to video if I saw anything else. In between the two lessons I really did not see anything. So this week I asked Alicia to come and video the run for me, she was not there for the first run but she got the second run. Lizzie was really doing some weird things and it almost looked to me like she was trying to pop her knee back into place??? Now I am wondering if that might be why Liz has been less excited about herding, perhaps it hurts? So, a couple of days off and a trip to the vet for Liz. Liz has always been my dog that physically is as sound as they come so it is really weird to be seeing things like that from her. I felt horrible when I saw the video, I am totally not watching my dog and I did not even see those steps, I had seen some things earlier, but... gosh if I had seen how bad that looked I would not have tried to change her direction that one last time....ALWAYS WATCH YOUR DOG, yep good advice.