Final answer:
Abdominopelvic cramping and minor bleeding experienced by a sexually active woman 3 weeks after the last menstrual period can be due to implantation bleeding, a sign of early pregnancy when the fertilized egg attaches to the uterine lining.
Step-by-step explanation:
If a sexually active woman experiences abdominopelvic cramping and minor bleeding approximately 3 weeks after her last menstrual period, this might be due to implantation bleeding. This is a common early sign of pregnancy, which occurs when a fertilized egg attaches to the lining of the uterus. If the bleeding is indeed caused by implantation, it typically occurs around the time a woman would normally expect her period and is one of the first noticeable signs of pregnancy.