Could this cracked drywall be caused by a shifting house?
I've encountered the same problem in my own home, although the drywall and ceiling seam cracks only showed up one at a time over the course of a few years, and most of them were present when I moved in. If more than one or two seams popped or nail heads started showing within a month, that seems like it could be a rather large settlement in your foundation, or some other structural problem.

I'd highly recommend getting a foundation specialist to your house sooner rather than later to see if your foundation might need piers to support part of it, or if some other repair is required. In my house, a total of 14 piers were needed along the side and front of the house to support one side and prevent further shifting. There were a few hairline cracks (even small cracks can be a bad sign depending on where and how many they are) in the foundation walls, and the corner of the house had settled over 2" before the problem was found.

The problem could also be in load-bearing structures (walls, I-beams in the basement, etc.), especially if it seems the problems are mostly towards the middle of the house.

The sooner you get this fixed (don't just let it go, even if you have to pay a few thousand), the better. You'll avoid more drywall repair later, and prevent further settlement and degradation of your house's structural integrity.