I'm sure there are lots of possible causes, but the only time I had a hound with symptoms similar to Beth's, it turned out to be a pinched nerve at the base of the neck/shoulder area. Took us ages to find the problem, as our regular vet couldn't find anything wrong, but eventually a greyhound specialist vet found it and some chiropractic sessions sorted it out.
If it keeps happening despite rest/painkillers, might be worth getting her checked over by a chiropractor - they often find things that vets don't IME.