First off, howdy and welcome to the forum
I agree that at first glance she does look pregnant, though it is entirely possible that something else might be going on. My money is on "bred" though.
Until you are able to get her to the vet and talk to him about what vaccinations she'll need and whether you need to de-worm her or not, about the best you can do is just make sure she's getting all the good food she can eat. Generally speaking, a healthy, well fed mare will make for a healthy, well-fed baby. I would avoid sweet feeds and grains, though, just good quality hay and plenty of it.
Or, by the way your pasture looks, just lots of grass on turnout.
(BTW, I am extremely jealous of your nice green pastures