She doesn't throw up the rest of the day? I was going to say that maybe it was b/c her stomach was empty and the acidity was getting to her. I know that Madison will throw up in the morning if I've given him some of their dry food before bedtime (like around 8pm)...the next morning usually really early, he'll vomit a little bit of it. So, I don't give him any of that anymore at night and instead give him some rice and turkey for his "bedtime snack". Occasionally if he has nothing in his stomach for too long of a time, he'll vomit, but then it's just fluid; not food.
If she's sleeping all night and doesn't have access to food (so she can't be eating it really fast and then upchucking?), and she's vomiting food, I'd definitely watch her to make sure she's not doing it other times too (a lot of times, they'll throw up food and re-eat it...gross, but Madison does it all the time unless I get to it first to get rid of it
he's fast though!)....and I'd give the vet a call at least. Good luck!