I had similar pains. I felt plenty of "period" cramps in early pregnancy the first time and thought it was just normal like from implantation or something. When I went for my first ultrasound for my second baby though it turned out I had a hemorrhagic ovarian cyst. I'd ask your doctor about the cramping just to be on the safe side (:
Normally cramping comes from the uterus stretching to make room for the baby. But like the lady that commented above said, just let your ob know so they can check your uterus/placenta next scan. I had a hemorrhage clot on my left placenta and I did get some cramping from it.