Thanks, Stone. She's been on Metacam for about a month, which hasn't made any difference, and tramadol (?) for a couple of weeks before that. The collar is a bioflow which she's worn for about 18 months. She's otherwise in good condition, and isn't overweight. I took her to the vets yesterday for the 3rd consultation, with a different vet each time, this one thought it might be a torn muscle. The suggested treatment is no exercise for 6 months; the other possible causes are a bone/muscle tumour, although he couldn't find any sign of this. She's started to turn her shoulder & upper leg outwards, so it's clearly causing her a lot of discomfort.
She's been a great little worker though, but I suppose the harder they work the more likely they are to get buggered up. An x-ray under general anaesthetic was also mentioned, although I 'm reluctant to put her through this unless it's essential. I'm not aware of any longstanding injury, although I've stopped her from jumping out the back of my jeep sometime ago, as this wasn't helping matters.