Strange might be worth going back and booking an ultrasound so they can make sure it's not ectopic is the pain in one side?
Have you confirmed with the dr that you are pregnant? If not that would be my suggestion and it could just be stretching pains not that baby is big enough yet but your body is starting to adjust for baby to grow!
When you say sever do you mean bent over in pain or like constant cramps. Have you had any bleeding? Also i would say call your doctor and try to arrange a visit.