Yeah, mine did this for a couple of nights a few weeks back... she was completely normal during the day, and I'd put her to bed, then ten minutes later I'd have to go in and get her out because she'd spewed everywhere and was crying... this from a child who has NEVER thrown up in her life, apart from a couple of newborn *bleurghs*!! So DH and I freaked out and thought she was dying or something, we took her to the hospital and they diagnosed a mild case of gastro, gave her Panadol for the fever which had magically appeared out of nowhere, and sent us home!
Apparently it was a mild enough case that she could ignore it during the day, and then when she was falling asleep it would get the better of her and she'd yack... a quick course of antibiotics, some Panadol at bedtime, and a few sheet changes later she was back to normal.

So, if your DD is functioning normally throughout the day, I daresay it would be something like a tummy bug, cold/flu/chest/throat/ear infection, kwim?? Nothing terribly major, but pop off ot the doctor to see if they can find out what the deal is and maybe prescribe her some antibiotics if it's an infection. If nothing else, the peace of mind is worth the trip as far as I'm concerned
Good luck, hope she gets over it soon and feels better!!