Is it normal for my period to last shorter than usual after a plab b pill? context: i am on birth control, took the plan b like 6 hours after sex, had some spotting, my period lasts between 3 maybe 4 days, this time lasted 1, my period came in time?

If you are on birth control then there is no reason to take Plan B. Birth control prevents pregnancy. Plan B is for people who have unprotected intercourse without condoms or birth control. Taking Plan B in addition to birth control will not cause harm but is not needed. Slight cycle variation from month to month in women on birth control is very normal