The first thing I thought of was what is termed as "Spring Eczema" in cattle... But horses don't get that specifically. In cattle it is caused by a toxin in the grass that causes photo sensitivity that makes their white patches peel off like a banana skin. Very very nasty and extremely painful.
I would hazard a guess that perhaps with the change in season recently, something she would normally eat has started to develop a toxin of some description, causing her to become photo sensitive... Very easily sunburnt.
First things first, get her completely out of the sun. Don't put a cover on her because that will rub on the sore bits, but she needs to be completely out of the sun at all times, stabled, put in a shed, whatever, but out of a paddock like yesterday.
Get the vets back out, I have a hard time believing that two different vets didn't think of photo sensitivity. If they honestly didn't seem to have a clue and you are a genuine horse owner not trolling the forum (not meaning to cause offence, but we do have those time to time so we can never be sure to start with) then get a different vet out.
And the second vet "guessing" that it could be a liver problem is just not good enough, did he/she not do a liver panel while they were there? Or was it a guess before bloods were drawn while you're waiting for results?
R.I.P ~ Bubbles - 25yo tb mare - 13.04.2011 ~ 8:30am ~ passed away naturally and peacefully in my arms