Weird periods

So I've been normal ever since I started and I was always 6-7 days. Maybe I was a day or two late here and there but that was the most. Never missed a period, I was never over two days late. It was always heavy for the first 2-3 days but it would lighten up a little. This summer, my periods have still been normal and on time but I'll get it and it will be lighter like the second half of the normal weeks. They only last like 4 days now. I haven't started birth control. I became sexually active back in November.  So now my period is on day 3 of being late. I took a pregnancy test that came up negative. I just really don't know why my body is doing this all of the sudden. I haven't felt good the last few days and I keep expecting to start, but I haven't. Any one know what could cause this?