I see from other posts that you've had her a few months & did not get a prepurchase exam. Also a remote history of founder. Have you been limiting her grass intake?
She could have arthritis or some other problem that was under control with bute &/or some other supplement or diet by her previous owners but has now appeared. It could also be something new but without a vet exam you may never know.
The money spent on a pre-purchase could have
prevented what you & her are going through now. Is she papered? If not do you really know how old she is?
I'll never understand why some people don't get a PPE. They weren't invented to get vets money.