If the sneezing comes when she lies down to sleep, then it should happen during the daytime as well, when she tries to lie down. Allergy has various ways to present itself. It also has a diurnal variation, means bouts coming at a particular time of the day. You could try a few of these things:
1. Start an anti allergic tablet or syrup and give this medication 1-2 hour before going to sleep.
2. Meet a chest physician and ask for a specific allergin test.
3. Have patience and wait for this to pass as she is already 11 and this may wean off on its own.