First she is a long time from being overdue. If she has not foaled by July then she would then be into the "overdue" stages but going close to a full year is very normal for some horses.
Over the years I have seen that how much bag a mare develops before foaling depends on what she is getting for feed. A mare on straight alfalfa will bag up sooner then a mare on grass. Both will produce lots of milk.
As long as she is not loosing milk (it would show on her legs) things are fine. I have seen both maiden and older broodmares who do not bag until the day before foaling or right after. She has developed a good milk vein so I would not be worried there either.
I had a pair of mares that I bred 4 weeks apart. They foaled the same night.
My first foal was carried one week less then a year and was a filly.