There are bees in my house, but something is odd. Can you help?

Today I discovered a yellow-jacket walking on the buffet in my dining room. Oddly, he seemed almost lethargic. After flushing him down the drain I looked around the same spot in my dining room only to discover more of them (about twenty) lurking around under the curtains near my two kitty-cornered windows.

While about five of them were fairly active, the majority of them were either dead or dying. What’s more, some of the dead ones looked as though they had been dead for some time (dry, covered in dust).

After I removed them all and cleaned up the area, I looked all around and only found a tiny, dried up nest inside the window casing (it looked abandoned).

Also, I haven’t sprayed for bugs since May nor has an exterminator been by.

Why would so many dead or dying bees turn up in one spot in my house? Do you have any experience with this?