Have you spoken to a vet? Given that you have been riding pretty consistently for a while and the horse is not unfit, she should not be showing lameness after three days of work. A sound horse ought to be able to manage three days of dressage schooling without any trouble.
I usually work my 22-year old mare in the arena (kind of Second Level-ish stuff) two or three days successively, then we go hacking, and she has a day off somewhere. In the winter, it's a bit more scatty because the weather can be dire and she might get extra days off, as we don't have an indoor. I don't usually do dressage for more than three days in a row because I get sick of it, but the horse is certainly capable of it.
A vet with an interest in physio or chiro would be your ideal person. But either way, I would start with the vet doing a lameness work-up, and if they don't find anything, move on to a chiro or physio.
The only other thing I can think of is that she may need more warm-up in the winter, but that doesn't explain why she is okay for three days, and then becomes unlevel.