From the area you described the hit in, and the color of the blood, it would have to be a lung. If not, you'd be getting a greenish brown mixed in I would think, indicating a paunch hit.

Were you able to find your arrow? Often, looking it over closely helps a ton, too... You'll be able to readily i.d. a gutshot deer from an arrow that's recently passed through some entrails; it'll probably stink to high heaven too.

Remember that a single-lung deeroften travel easily upwards of 300 yards as well. Keep on going in the line that the deer you shot was heading; maybe the doe you saw coming back wasn't her. In fact, let's hope it wasn't. I'd spend a couple more hours tomorrow going that direction. Then, if that proved fruitless, I'd come back and go in the direction you saw the buck running the doe.

Best wishes in finding her.