Welcome to my life :). I have tight stomach all day almost every day, worse when I "exercise" (which these days is mild walking) or am just up and about too much. My OB attributes it to BH contractions, the fact that I had a pretty tight abdomen prior to getting pregnant, and irritable uterus (because I basically have BH contractions that don't really go away for much of the day). She's not concerned, particularly, because my cervix is tight and closed.
Obviously, get the opinion of your doctor, but it sounds similar to my experience.