He ejaculated inside me yesterday and now today i'm started too have cramps could this be a sign of pregnancy?

Too soon to tell: It is unlikely that the current symptoms are due to pregnancy this early. It could be other things, however. If symptoms persist, see you doctor. It takes several days before you can tell if you are pregnant or not. So, its unlikely your current cramps are due to pregnancy. This does not mean you are not pregnant and follow up is key.