Jennifer, I would have her checked. Several times we have had mares bag up here VERY early, and it turned out to be placentitis. Not to worry, it is treatable, and baby will be fine during the treatment. These mares have gone on to produce healthy little ones. It's always better to get the vet's opinion, if you're sure about the dates of the breeding and expected delivery date.
Keep us posted!
Can you post a couple of pictures of her: a full side shot down at her level, and one standing at her rear looking down her sides down at her level, so we can see how baby is riding. That will give us an idea of how she's looking.
Are you sure of the breeding dates, or is she pasture bred? Sometimes, when pasture breeding, the mares can let the stallion breed them when they're already pregnant, and then the dates are hard to know. If you observe a breeding and work from that date, but she has already settled, your dates will be off.
So, who is this pretty little lady? And who is she bred to?