Call the doctor. I had the exact same thing with DS1, when I was around 20 weeks. I started having pains, that I thought were gas pains, but they wouldn't let up and I noticed they had a pattern. We went to the hospital and I was having contractions. They gave me terbutaline to stop them, ran an IV because I was dehydrated- and I ended up diagnosed with an irritable uterus, and I was put on Procardia.
They never officially diagnosed me with pre-term labor, and my cervix never changed, but I did end up in L&D 2 times after that with more contractions. It's worth it to call and double check.