That's OK I just want to be sure.
The most common cause for the sudden onset of a severe hind limb lameness, but still bearing a bit of weight on it, is a hoof abscess (in the sole) or gravel (in the white line).
This is caused by either a sole bruise or a small piece of dirt working its way up the white line between the wall and the coffin bone. It causes a picket of pus that starts to get larger, which puts a lot of pressure insude the hoof and makes the horse VERY lame.
The reason I was persistent in asking if she was NOT putting any weight on it at all is because that could be more of a sign of a broXXXXX XXXXXmb. I have seen hoof abscess horses that hold the leg up and don't put any weight on it, but it is not common.
If you examine the coronet band all the way around the hoof even over the heels, you might see a place where the coronet band looks red or purple, or swollen. If it is a gravel, then this is where it is the pus is going to break through.
The good news is that these look horrible but are not usually life threatening. Once diagnosed, I usually prescribe bute, soaking the lower leg in warm water and Epson salts, a tetanus booster and stall rest. The affected leg may swell some until the abscess bursts though. If it is in the sole, you vet may or may not pare some sole out to find the draining tract.
Once the abscess or gravel ruptures, which usually only takes a few day but can take up to a couple weeks - the horse gets much relief and is sound again within a few days. Sometimes I add antibiotics AFTER the abscess ruptures, but this is not common unless the rupture hole is huge.