Lame Willig; Leased (?) Mercury

Willig went lame over the weekend. When I went out Saturday, he was a little off, but when I went back out Sunday he was a cripple. His right shoulder is knotted up pretty badly, but I can't find any warmth, swelling, or tender spots. And I've poked all around his feet and it doesn't seem to be coming from his feet. The lameness isn't the big problem though; it's the stall rest. Willig HATES being separated from the other horses when they get turned out. Even with all the activity Saturday with lessons all day, he was a total brat - in pain and pissed off makes for a hard to handle Willig. He did, after about an hour, settle down and be a sweet boy.
In the meantime, Mercury is on his way to a partial lease. The girl who is interested in him has had a couple lessons and is more comfortable with him being a good choice. She was out Saturday and asked if she could do anything and I said if she wanted, she could groom Mercury, and she groomed him for like an hour, and then took him out to eat grass. He was so shiny and clean. That's exactly what I was looking for - someone to dote on and love Mercury. So hopefully that will all work out.
But for today - another call to the vet, who is probably wondering if I fell off the face of the earth after my December-February earth shattering vet bills, and then nothing (thankfully) for March.