Leg issue

Question
Hey, I wanted to ask for your help. I took my horse (mare) for a ride today and at the middle of the lesson, my instructor and my father told me to stop the horse because it seemed there was a leg she didn't really lift off the floor, it seemed that she was sliding it in a way. I tried to make her trot on the side of her "damaged" leg and it got really obvious something was wrong so I stopped. I tried cold hosing her and walked her a lot too but it didn't get much better. My instructor supposes it has to do with the stifle, but my dad and I realized that the fetlock joint was swollen. What should I do? I'm talking about her left hind leg.

Answer
Hi Karine.
  There are a lot of things it could be. I know it can be expensive, but I really think she needs to be seen by a good vet. I am concerned that there is something neuromuscular going on, rather than a simple strain or injury.
  Best of luck, and please let me know what you find out!
Anne Stepien