I know just as little about pregnant mares as you do, so I'm probably not much help. If I could guess I'd say yes, because her belly looks more than just full of hay and the udders look plump compared to a mare lesson horse I used to ride.
It would be especially helpful for you to look back at some older photos of her from months ago when you got her. Compare/contrast her shape, and maybe it'll be easier to notice the difference.
As people have said, gettin' a vet is the only true way to know. I vote that yes she is, and I hope that all goes smoothly for you and you can sell the foal so that things don't become too stressful for you. Goodluck!