thank you both. Farrier has just left, feet are fine. To be honest, I'm not really sure what exactly caused her to become anaemic. My vet didn't really offer much explanation as to what caused it. Reading up, I found the following quote:
"Causes of anaemia in a horse or pony include a shortage of red blood cells or haemoglobin through haemorrhage (bleeding), infection, weakness in the immune system, redworms, bots, liver or kidney damage, tumours, poor diet (a diet lacking iron or B12 causing a deficiency) or diseases such as equine infectious anaemia."
I can discount redworms and bots pretty safely. She's wormed regularly and my vet confirmed that our wormer protects against redworm. Also she is on a vet recommended diet because she was overweight, so he also confirmed her diet is appropriate. She has had a lot of vet attention in the last 6 months, and I think they would probably have detected haemorrhages, tumors, and kidney or lung damage.
While i'm not 100% I would probably guess an infection, because it seemed to be quite sudden, especially her mucus membranes changing colour. I'll definitely ask my vet when I pick some iron tonic and schedule her follow-up
~The most important lesson my horse ever taught me is how much I have left to learn~