Missing a period

I’m (f24) just wondering what are reasons for a missed period other than pregnancy? My period is already 4 days late and it’s never been this late; max two and a half days. I took a pregnancy test and it was negative, i think this might be a health concern and i need to see a doctor asap, do you have any thoughts on why it could be so late and i dont have any cramps or symptoms like I usually do before it starts?