Chronic abdominal cramps 3rd trimester

I'm 37 weeks pregnant. Yesterday it snowed and I shoveled the driveway. I must have overworked my body because I've had chronic lower abdominal cramping ever since (18 hrs and counting).

The cramping is horrible when I stand or try to walk to the point that I can barely walk anywhere. But when I sit or lie down, the cramping becomes almost nonexistent. Anyone have something similar happen?

I know it's not contractions because my position causes the pain to get worse/better. Baby is moving like normal, so I'm not concerned about her. I called my doctors office and a nurse told me that unless baby stops moving or my symptoms get worse, I don't have to come in. I'm just curious if anyone has had this happen to them or if anyone has a theory on what's going on with my body?