Vanessa, you will have to stop painting and get the paper off the ceiling totally. Wallpaper is held on with a water soluble glue so it can be removed. Your paint has water in it. A wide putty knife is probably about the only tool you can use and this will be terribly messy.
Preparation is part of the painting process and part of preparation to paint is washing the walls--and ceiling. If you washed the walls and ceiling, the paper probably showed signs of releasing. This should have been a clue for possible problems.