To clarify the terms that you are using, the cycle is the time between the first day of your period in one month and the first day of your period the following month. The cycle can’t last 7 days... but your period (menstruation) can last 7 days. If the duration of your period (menstruation) shortens from 7 to 3 days, it is not a sign of pregnancy. It is possible that 3 days of bleeding is not a real period but may be spotting or false menstruation. False menstruations happen when ovulation has not occured (you did not produce an egg that month). You can take a home pregnancy test to confirm or exclude pregnancy.