If this helps anyone, i started my period today & it was lighter than usual so i took a pregnancy test, & i got 2 negative tests!!!
It sounds like you had unprotected sex on day 11 of your cycle, which is very dangerous.
PlanB can make you bleed and spot and make your cycle irregular, and that's likely what's happening, but you were having sex within your also likely fertile window.
Do you have access to birth control? If you're not pregnant, now would be the time to investigate a more reliable system.