Has she been dripping any milk - or have you seen wax on the ends of her teets? Is her rear end very soft and her tail you are able to move around with no issues? Is there a visible V from the her front legs to where her withers are (foal has moved into position)? I would keep an eye on her then. Some mares carry a bag for a long time until they finally seem like they are going to explode - but you should see either milk dripping or wax, etc.
Good luck! Happy foaling!