7 days late

My period is 7 days late and I've taken three pregnancy tests so far and they were all negative. At this time in my life I'm not yet ready for a child. Is it just stress from school? Or maybe being around my friends all day so my period is changing to match up with theirs? I took a test Saturday, Monday, then today (Wednesday). Any chance one of you guys have some insight or experience with something like this?