I had unprotected sex i knew my period would start the next day and it did. i took the plan b pill on the same day will it effect my cycles length?

Could: Plan b does have hormones in it so it could alter your cycle , but only temporarily

Happens sometimes: Your cycle length may be slightly altered but will most likely go back to normal within the next 1-2 cycles, if not then consult your PCP or GYN.

May: not since your period is starting the next day. Though plan B can cause bleeding and spotting