i think Chantal is right. I get it too. Sometimes its worse than others and sometimes I have to stop and stand still for a bit, then it goes away.
Could be your uterus stretching. I know at times it hurt me and felt like I just did crunches. I actually am starting to feel the same thing again at just about 30 weeks. I know my baby is head down and must be getting bigger, so that's why I am feeling it probably. Just be careful how you move and what you lift.
~chantal~