Bonnie,
I have been reading your thread. I am curious. Could you please take some pictures from straight behind your horse that clearly shows both sides of the belly? (eye level with the belly is best, but do be far enough away to not get kicked!) The tail can be left to hang naturally. Take them for a few days.
The reason I ask is that when a mare is in foal, often (but not always) by the time they start to bag like yours, you can see the foal change what side it is laying on. So the belly is not perfectly symetrical and the closer to foaling it gets, the more obvious it gets. Sometimes one side will look like it was slightly flattened out even.
Another thing to look for is that the foal will drop and move back when it is very near foaling. Again, on a maiden mare this can be hard to see. But from the side, you see the belly bulge get lower and then it will move towards the hind quarters when foaling is emminant.
Keep a close eye on the tail head area (where the tail joins the rump). The signs can be subtle in a maiden mare, but their will be changes in the firmness to either side of the tail when foaling is emminant. This is where the mare will get to feel like jello. The tail head will move easier when you lightly press there. Also, pick up the tail and notice the resistance when you do so. As foaling becomes more emminant, she will have less strength to resist. Of course this again will depend on the individual as I have some horses that are really relaxed when I am playing with their tail. But you are looking for deviation from what is normal for her.
Another thing to try.... If she is close enough to be bagging, You may be able to feel the foal kick if you "bump" the baby. Check at feeding time. Give your mare a light belly hug just behind the belly, but infront of the hips. (be gentle and be careful as this area is sensative) Sometimes you bump the baby doing this and it will kick you. It will be noticeable and will be on one side or the other.
If this mare were mine, I would be watching for all these signs and the ones that LisaF posted about as a general feeling is that the bag is more than just hormones for a maiden mare.
And like the others, I have a mare that was actively showing to a gelding until she foaled. She was a maiden mare and didn't know what to do with herself and would have let him breed her if he could have. Little hussy is right! LOL.