If you are taking her outside, and she is going when she is there; then it does sound like it might be an excited reaction.
one thing you can try is putting her in a crate overnite, and then making sure she is out long enough to get all of her chores done when she goes out in the morning.
If she then comes in and still has an accident in the house, then I think it is either a physical problem, as was suggested. or it is just excitement.
They make doggie pants/diapers that you can try if she is doing this a lot and it is being a problem with having to clean it up a lot.
