Constipation at 31 weeks pregnant...?

It had been over a week since I had a bowel movement and I was very uncomfortable. However, I had tried Colace, Milk of Magnesia, and even Glycerin Suppositories, but NOTHING worked. Well yesterday and again this morning I had a small bowel movement that was kind of like diaherra, but yet it was still hard to get out (almost had to strain) and there wasn't very much in consideration I hadn't gone for over a week. I know this is almost too much information for some people, but not going to the bathroom like this worries me a bit. What could possibly be the problem?