You may have gotten stuck with drywall from China - it's been a huge scandal, and if you got unlucky (tho' dampness could be a factor I suppose, or paint surface incompatibility of some kind) then you need to Google re Chinese drywall and find out how to get recompensed. I think there may be class action suits and who knows what going on. Or you could at least get in touch with whoever sold you the drywall.

"If there was water damage, wouldn't the sheetrock be soft or the paint stained?"

This does NOT sound like the Chinese drywall problem.
It was caused by suplphur coompounds in the drywall out-gassing.

I would suspect water damage.

Have you checked to see if the wall is soft?

Water damage might appear without making it through the paint and causing staining (especially if the water is clean).

Try pushing on a 'bubble' and see if there is anything behind it, and if the drywall is solid.