Should cramping worry me?

I'm 18 weeks along and am occasionally having feelings in my abdomen that are very reminiscent of menstraul cramps, although there is no spotting. I haven't felt the baby move yet (to my knowledge) and I'm just wondering if this is a normal occurrence, perhaps this feeling is the uterus expanding and adjusting versus cramping? I have noticed a significant increase in the size of my abdomen so it would stand to reason. Thanks for any helpful input you can offer :)