Pending a vet appointment and on stall rest for now -
She is a 4 year old big Warmblood and has been growing a lot. Started under saddle about 18 months ago and has been going very well. At first was not picking up right canter lead and was worse / weaker to the right, but improved greatly. Then she stopped taking her right canter lead.
Her right hind leg looks strangely weak. She is dragging her toe, and sometimes appears to cross her right leg under her middle when trotting or cantering. When walking down hill, her right leg and hip really looked "off" from behind and she appeared to be down on her pastern on that leg as well.
She does not seem to be in pain, no swelling, heat, or known injury, but she has been pretty lively turned out in the past so she easily could have hurt herself.
Best to get an exam, you may need to have the vet watch you ride. Does the fetlock drop? Have a video? Taking a video of the problem in these not quite right situations can be helpful for the vet also.
I was taught by my first great vet "Always start at the bottom". I would first make sure that her feet are well balanced . An unbalanced hind hoof can cause all kinds of hip/stifle/hock problems and even a stiff sore neck, and reluctance to do something you ask. And yet, the hoof itself will not show lameness or soreness, just higher up.
Starting at the bottom and working up is playing by the odds, but given the breed/age/signs, I'm with the stifle group.
I'd be suspicious of OCD.
No doubt the DVM in question will perform a thorough exam, though, rather than jumping to a conclusion without looking at the beast.
"It's like a Russian nesting doll of train wrecks."--CaitlinandTheBay
With those symptoms, in addition (obviously) to a thorough exam, I'd want digital xrays of the neck.
Yes xrays of the neck with a thorough neurological exam by someone who is a specialist. Most vets in my experience (even very good surgeons) don't have a clue how to properly test for and diagnose neurological problems.
Seriously, after nearly four year long wild goose chase which included a stifle surgery and year of rehab, I wish I would have had one person mention neuro problems as a possibility...