You can’t be 3 weeks pregnant! The pregnancy age is counted from the first day of your last menstruation. You can’t be sure if you are pregnant or not 3 weeks after beginning your last period because the most sensitive pregnancy tests can detect pregnancy 10 days after conception, at the earliest. That is in the 4th week of pregnancy. An ultrasound can detect pregnancy in the 5-6th week. Cramping could indicate nothing, but vaginal bleeding is usually a bad sign during a pregnancy.