She's doing better, thank you. I forgot to update this thread! My vet called back and her blood test showed that she's anaemic. Her red blood cell count was way down and she wasn't able to produce much haemoglobin. That was pretty much draining her energy and making her really drowsy. I kept her in her stable with a small run for about a week and have been giving her a vitamin and iron tonic to help, she'll need to take the tonic with her feed for a month or two and then i'll arrange a follow-up blood test to check that her blood cell count is back up to normal. Other than that the vet said she was pretty ok. Having turned her back out and with her on the iron tonic she seems to be picking up well, so I took her out for a gentle hack on monday and she behaved normally, so we'll see how it goes.
I'm pretty relived, I was worried it was laminitis or foot trouble. My farrier is due in an hour or so, so he'll give her a quick foot check, but i'm pretty certain laminitis isn't the problem now. My vet did say it was a good guess though! Haha.