Thank you everyone :)
The problem is, in the pasture she gets MUCH worse. In the round pen, no improvement.
I've sourced some mulch bedding - it's not great but the best I can get at short notice - but they can't deliver until Monday so she's in the box that doesn't shut off from the (sand and grass) yard (that particular box has a broken door into the yard which we have removed - the other box doesn't HAVE a yard). She can walk around, and does a lot, can get up a trot for a few strides but can't canter. Cantering makes her go horribly horribly lame so I don't want her doing it at all if I can avoid it.
Vet is coming this arvo and they know it might be a tendon... we initially thought it was a foot issue, many of which I can solve myself (thrush, seedy toe, laminitis, greasy heel etc - she did have thrush and a little bit of greasy heel when she first went lame but those have since come good and she's still intermittently lame) but then the tendon blew up the first time. So into the round pen she went, but she could still run around and get up a canter, so she would improve a little bit then go for a canter and go lame again.
The problem is that this is very intermittent, so I'm not really sure what to think, but there is definitely some heat and swelling in that tendon. If this is a tendon thing it's a minor tendon thing as such injuries go. I'm hoping it's just inflammation or a strain... a tear might end her jumping career before it's started.
Here in Aus, the advice seems to be distinctly different, as my knowledge based on the experiences of multiple friends is that box rest is what is recommended by nearly every vet. Box rest and hand walking, then box rest and hand jogging, then box rest and lunging on large circles, then pasture and lunging on slightly smaller circles, eventually working up to light work under saddle - but she's not broke yet so under saddle isn't even on the cards.
She has HEAPS and HEAPS of time to heal properly... won't be broken in until next year at the earliest and at this rate if she keeps maturing as slowly as she is probably the year after that. She is 2 but doesn't really look it and is growing like a weed.
On the plus side we had a big panic attack last night and this morning no sign of lameness at the walk, so being confined has done her good... she was definitely lame yesterday. Might not be a tendon thing but whatever the heck it IS, confining her has helped. And did help before.
This thinking it might be a tendon thing is actually pretty recent. After the foot issues came good we thought it was a muscle thing because giving her a hamstring massage she would kick like a demon - not AT us but expressing her discomfort - and she's not a kicker, nor is she ticklish in that area. Then the muscle came good and she was fine for a while, then she went lame again, the tendon swelled, and the muscle is still fine. Hence, through process of elimination (and the tendon swelling up) we have assumed - yesterday - that it must be something to do with the tendon.