Lame :(

After my great lesson with Cupid last week, I was really excited to keep building on it.  Unfortunately the following day he was totally lame on his left hind leg.  He wasn't lifting it all the way and the toe was dragging on the ground.  There was no swelling, heat, or tenderness anywhere.  At first I thought maybe his foot was bruised, since he got trimmed the day before the sole was more exposed.  But it looked more like it was coming from the stifle.  At first I thought the stifle was just locked, after doing a bit of reading online I tried backing him up to see if that would unlock it but it didn't help.  The vet came out today and also suspected the stifle was the cause but he could not determine what specifically the issue was.  The options were to either put him on stall rest and see if there's any improvement in about 2 weeks, or more diagnostics.  I opted to have xrays done so the vet will be back tomorrow.  I'd rather know for sure what the cause of the problem is, and then have a better idea how to treat it.  So fingers crossed we figure it out tomorrow, and it isn't anything serious!