This is a tough one with out actually seeing it.
some ideas...
Were the joists wet/moist when the drywall was installed? Moisture might have finally made it's way through. The drywall will suck the moisture out of the wood. Im sure its not wet but it might of softend the ceiling paint a little and made it darker.
Is the home settling and put a little pressure on the ceiling were it might be bulging and you seeing shadows from it?
Looking at it from the floor gives a different perception than if you get a ladder and look at it real close. It might give you a better idea of what is going on.
Just a thought, I might be way off.
It might just need a couple coats of premium paint. Use a primer sealer on the dark areas first.
Joe