There is always a chance of pregnancy when ever you have sex. Just because he didn't intentionally come in you doesn't mean there wasn't any precum. Not sure if you used a condom or not. And since you did it the day your period was supposed to happen the chances of pregnancy is really low but still possible... How long ago did this happen. Did it happen last month or this month?

No by having sex on the same day it might push your period back couple of days, that what mines do...as long as u have your cramps your fine ..if it take a bit longer then expected you could call your doctor and ask