I'm not close to being an expert but here's my opinion:
Did the area where the cut feel warm before riding? With the farrier saying she was showing signs of founder in the other leg could be that she was favouring the hurt leg there for the other one got too much of the horses weight on it and had too much pressure and so there wasn't an even weight distribution. As for the weather she could be off because of that. Just keep watching your mare check for heat in legs, thrush more signs of founder, again I'm not an expert but that's just a little input! Hope that helped you out a bit!
Practice like you've never won, perform like you've never lost