I'm really nervous but I don't think I should be

So I'm 15 and I have only been sexually active once last July. We where safe and my period was on time. Now I'm with my new boyfriend and we have bee dating for 6 months and haven't hand sex. We have had many make out sessions and there has been some dry humping along with it. My period is like clock work and it is always on time. This month I'm 3 days late and I don't know why but I am really scared. I'm not on birth control and I didn't just get on birth control. He has never been inside of my so I can't be pregnant, right? Does anyone know why I am so late