Plan b questions

Okay, I did a dumb move. I had unprotected sex for the first time in forever with an ex of mine & I feel stupid as fuck I should have made him put on a condom, but that dick had me hypnotized he said he didn't cum in me but I'm still don't have any peace in my mind so Ima buys plan b tomorrow since it's night time the thing is that my period starts Monday & I just want to ask if it too late or I'm good idk I'm new to this my question is will plan b still be effective even if my period starts Monday Though I'm a grown-ass woman I'm scared to ask mom I don't want her to be disappointed in me I don't have unprotected.. hell even sex often. This sooo stressful I promise I won't do that again.