Do I have to replace my drywall or can I just paint?

If it is just the paint, sand it, prime it and repaint it. But, if you have bulging and the drywall is wet inside, cut out the damaged area, patch it with a piece of new drywall, tape it, sand it, prime it and repaint it. All dependent on the level of water damage.

Hello, you will have to at least replace the drywall in the area where the chunks are. Once you take that down you will be able to tell whether it has gone further.

If there is no damage to the drywall (soft or spongy) then you can scrape off the paint. Sand smooth. Prime with Kilz and paint. Primer will seal any stains.

You need to put a primer on first if the paper on the drywall is still in good condition. Then put your finish coat on which should be a ceiling paint.

You should also check the insulation above it. Often times there is mold and it will need to be replaced.