Missed 2 periods?

So I missed two periods, in November and December, but only had the lightest amount of spotting. I thought I was pregnant, because I had unprotected sex when I was ovulating, but took three tests a week after my first missed period and they were all negative. But I still missed my period in December. I started my period on Sunday, and it's back to normal. What could be causes of this? Has anyone else experienced this?