Why am I late?

I am currently 3 whole days late on my period. Its starting to get to me rn. I had unprotected sex about 9 days ago. (He did not cum inside of me, but Ik it's still a risk.) I've been cramping a whole lot like I usually do but still no period. I was reading you shouldn't take a test too early or it might be wrong result. I'm too scared to anyways. What are reasons why it is so late? How long should I wait before getting a test?