Last week, our 8 year old mare started refusing to pick up her left lead, which is very unlike her. We thought she could be sore so got her chiropractored/massaged on Thursday before competing Saturday and Sunday. Because we got her worked on so close to competition day, she ran on banamine to help with the soreness and ran great. On Tuesday, went to lope circles on her and she refused again. Won't pick it up at all. Our lameness vet doesn't have an open appointment until June 1st. She's had multiple treatments daily with the beemer. She was fitted for a saddle last fall and has the correct size. Any ideas? 

Sore hocks are the most common cause. Usually they dont show up on a lameness exam. Joint injections usually fix it, chiro will not. 

EPM

When my mare struggles with the left lead, and refuses to take that lead, it is her EPM flaring up. 

If she's working good on pain meds but not without it's a pain issue.  Start the tricky process of figuring out what hurts and why.

If you could at least post a video of the horse moving, or maybe even making a barrel run, that might help.

To me, getting adjusted on Thursday and running the horse Saturday isn't "so close". The banamine likely covered up whatever pain issue is bothering her.

Right now, with the information provided, it's an absolute guess in the dark.

Most common problem with barrel horses are hocks and stifles but who knows. You'll just have to wait for the vet visit.

I had this happen in one situation due to a torn suspensory. I had one that started getting a preference for being on a particular lead which ended up being pedal osteitis after a wet spring, but it had some other issues as well. Without more information, it could be any number of issues.

Have you checked your saddle fit? My mare stopped picking up her left lead because the saddle was pinching her behind that shoulder. 

Her saddle was fitted last September so I don't think it's that. Her hocks were injected in the beginning of March but she could have slipped in all the mud around here and tweaked something. She's been kicked out to the pasture for the last week and our vet was able to fit her in on Thursday so we will be doing a full lameness exam and chat about EPM testing as well. She's been tested before for EPM but that was a few years ago.