Do this mean miscarriage ?

Maybe you got pregnant later than you suspected and that is why it is showing gestation of 5.1 weeks...In an ultrasound this early you cant see very much and wouldnt see a fetal heart beat yet....As long as your hcg is doubling every 2-3 days and you have a follow up on your ultasound, it just sounds like a very early pregnancy...However if you have any bleeding along with cramping then that is a good indication of possible miscarriage....Good Luck!