Why is my period playing games with me?

So I have always had really normal periods. Never missed a period, if it was late it was always within a few days. Well this month I'm late by 16 days, it's as if my period has disappeared entirely. Took 3 pregnancy tests all different brands and all came back negative. So where is it? What could possibly cause me to skip a period? And should I see a doctor or be worried about it?