What Causes Nausea Upon Bending Down?

I get nauseous when I bend over cleaning house. I thought is was the chemicals I have been using, but this time, I was just bending over to move a few things, when I was sweeping the floor - no chemicals. It happens every time I clean house. I even, jokingly, told my friends I am allergic to house cleaning. I am not really light headed, although, occasionally that will happen. It is mostly nausea. I often have to lay down until it goes away.