I'm 23 weeks and just found out I was pregnant at 21 weeks. When I was on depo provers birth control shot for 13 yes and was told repeatedly by docs that I was sterile. This is also my first pregnancy. I believe the cramping is normal. I have it almost everyday I'm sure its just the uterus.
Sounds like growing pains as long as there's no bleeding. If it continues and your worried seek medical advice just to be sure and put your mind at ease