What Did My Horse Do Now?

So two days ago when I took Telly out for a quick bareback ride I felt that he was a bit more stiff than usual. He was not too bad but in my head I felt that things were not looking up. So I wrapped him for the night and hoped it was just a bad day for him. Yet, come morning upon turnout I could tell he was doing no better, actually worse. He was noticeably having a harder time walking though I could not figure out which hind leg it was. But I turned him out hoping it would be good for him. I unfortunately did not end up grooming him or checking him over that night, so I cannot say if I missed an early sign or not. But this morning I took him out to rinse him off and clearly noticed his left hind limb swelled. There was a bit of swelling down the back of the cannon bone with much more throughout his fetlock joint. It was mildly warm but upon some squeezing and rubbing over it there was no huge reaction from him. I did notice he was keeping the weight off it as I rinsed him down and cold hosed the leg for 10min. I gave his legs a good towel dry and then wrapped them all. I also gave him 2 grams of Bute and will be keeping him in for the day. He is definitely having a hard time walking on it, and he is dragging the right toe. I am praying he did not tear something or do major damage to it. He is already going through so much. I am debating calling the vet to come out and probably have an ultrasound done. Might see how it is tonight and if no better than call in the morning.