Late period

Hey guys!

My period is 10 days late and usually it’s pretty consistent. Sometimes being only a couple days late. I had sex two weeks ago (he pulled out though) and I’ve taken a pregnancy test, which was negative. I’m under a very minimal amount of stress. Its hot as hell here (idk if that effects it in anyway) and I’m only 20... any guesses as to why I’m out of whack??

Thanks :)