It could be just growing pains or implantation pains (when the embryo actually attaches itself to the lining of your uterus). I remember my OB telling me that women who are thin tend to feel more "crampy" pains as pregnancy progresses because there is less room for the baby to grow...except out! So it can be uncomfortable and even painful as the muscles and ligament stretch to accommodate that beautiful baby growing inside you. :)
Also, try drinking LOTS of water and put your feet as often as possible. I was constantly having contractions at the end of my pregnancy (I even sent myself into pre-term labor!) because I wasn't drinking enough water and I was getting dehydrated. When you dehydrate it makes your uterus more sensitive and can start cramps/contractions. Try this: my OB told me that you should drink so much water that when you go pee it should appear clear as water in the toilet (no yellow...LOL).
Hope this helps...Good luck with the rest of your pregnancy!