Her bag is hard because it's getting ready to produce milk it is normal, if it's getting bigger it should be warm and hard. Nothing to worry about. It usually starts doing that about 2 to 4 weeks prior to delivery, some mare start sooner some later. There is no exact signs and symptoms when it comes to foaling some mare exhibit all the signs some a few of the signs and some none of the signs. I know this doesn't really help you. I know your stressed out, as we all are that are expecting foals, lots of sleepless nights during foal season.
The foal will be delivered when it's ready to be born, not before and not by the calendar. I have mares that go way over the projected due date and mares that deliver before.