If you are correct on your dates, then there could be an issue with a calcium imbalance.
I had a friend breed her mare to our stallion and about this stage, she bagged up, started showing signs of being in labor, and was also obviously very painful. The vet ran tests (not sure what?), prescribed meds to prevent birth/retain pregnancy and calcium to balance out her vitamins and minerals. 3 months later she delivered a healthy, beautiful stud colt with no complications...
cute mare. Looks to be bay with a pangare (spelling?) gene. Stallion looks to be chestnut (ee) but could be a silver bay (don't think so?). If they are bay and chestnut - you could have a black, bay or chestnut foal. If the stallion is silver bay - then you throw in the silver gene as well - which will show on a black or bay base coat horse... I'm not sure how the pangare gene works - haven't read up on it much.