I have had this with all of my kids. For me, it would be at night. I'd be in bed and then, suddenly, I would just be doubled over in awful cramping pain all throughout. I'd go downstairs, clinging to the walls so I wouldn't fall since the pain was so intense. I would then turn on the TV and try to find something funny to watch to take my mind off of it. Sitting would hurt too much, so I'd try walking around. Sometimes, I'd feel a little better. Eventually, I would go back upstairs and try to sleep it off. I'd wake up in the morning, feeling completely normal.
With my second-born, I didn't know I was pregnant for a while, so I kept popping Midol, thinking I was about to get my period. Oops?!! He's perfect, though.