18 weeks....advice?

This morning when I got out of bed I had some pain in my lower abdomen. Sometimes this happens when I've slept for a long time but it goes away shortly after I pee. It's been 4 1/2 hours and it's still there. It is relieved when I sit or lie down, but is back when I shift/stand/walk and worse when I try to go #2. Anyone else have experience with this?

*My OB won't be available until tomorrow morning. I can't really afford a visit to the E.R....especially if they are just going to tell me I'm constipated or something.