Is that her in the picture? If so, here's my thought because what you describe is exactly how my TWH started out when the insulin resistance exploded on him - then he went from being overweight to dropping weight very quickly.
She is overweight (if the picture is the horse you're talking about). If she is in the early stages of insulin resistance, believe me it is what is wearing her out and making her lie down all the time. To repeat myself that is exactly what happened with my horse.
I would try to get the vet out ASAP and ask his/her opinion as to whether or not she could be I-R. The vet can do an ACTH test and will be probably come back "levels normally high and inconclusive". Whatever you do, if the vet does suspect a metabolic issues, don't let them do a dexamethasone test (dex test).
My horse has been formally diagnosed with i-r for three years and has never foundered. He is still out on pasture 12-14 hours daily but is on a very strict diet.
What is she getting for feed? My thought for now would be to take her off all her feed except for hay and vit/min supps --- meaning no feeds that contain oats, corn, or soy just in case she is on the verge becoming a Type II diabetic, which is what I-R/EMS essentially is.
She may even have a UTI for all I know as I have all geldings and things are pretty basic with them.
I hope this helps and please let us know what the vet thinks. It's possible I am way off base, but for sure, something is wrong for your horse to be lying down as much as she is.