I should be 10w and 4d pregnant but the u/s showed that I am only 6 weeks and doctor could not see the heartbeat. Have been told to go back for u/s in one week. I am not bleeding yet. Last period which I had was 13th of February 2014.
And my periods are pretty regular 28-25 days.
Is it possible still not be bleeding.

They usually measure the gestation time from the start of your last period and adjust for size/weight. There is a chance that the fetus is just small, which is why they're wanting a second Ultrasound. If nothing has changed and there's no sign of a heart beat then they'll probably induce your body to pass the fetus. It could be that your body hasn't realized that the pregnancy isn't viable any more, but I would be prepared for that eventuality. I am so sorry.