Yes, with maiden mares it is hard to tell sometimes. My maiden mare (first one) foaled with NO BAG whatsover and even after she foaled she did not get a bag but did on her foaling date.
Typically the nipples will fill with milk and become fuller and also point straight down and out a bit. I just had a mare the other day who had a rather firm but not big bag and no sticky milk at all on Monday night. Tuesday night I checked her and her bag was tighter (but not bigger) and her milk sticky immediately. She foaled the next night. Some mares/maidens are text book.
We have a name we call this: Mare Stare, Foal Watch etc., most of us cannot tell exactly when they will foal. Not even the vets can tell you. There is a wide range of when they can foal. Given your dates though it could be anytime as the baby can move up into the birthing canal in a very short period of time, this causing the bag to change, milk to fill the teats. The V will disappear as well just prior to foaling as foal moves upward into position.
Good luck, please show us your photos when the baby arrives!
PS: I am no expert, just my observations of foaling over a period of years.