cramping pains in early pregnancy

light cramping is very normal in pregnancy as long as there isn't bleeding. The cramping is due to your uterus streching(to make to room to the growing fetus)

what about the fact that no baby was present inside the sack

4 weeks along is not long enough to show the baby, it is only a cell/zygote at 4 wks along. At 8/9 wks, you should be able to see the amniotic sac and a tiny baby and short umbilical cord attached to the placenta in the sac. Mine was the size of a walnut (the sac, placenta, umbilical cord and the shape of a tiny baby). I have had 5 children and 3 miscarriages and pregnancy cramps are similar to period cramps, but with no bleeding. Some women bleed a bit at times during their pregnancies, mostly early on, usually it is nothing bad, but see the doctor to cofirm it. My first 3 months were so painfully crampy that it scared me, 1st pregnancies are the worst b/c your uterus has never stretched for a baby before. Best wishes for a healthy pregnancy!