Late period, not pregnant. Any ideas?

I came off depo last July-ish and my periods started again January this year with no trouble. I have been pretty regular, averaging about a 32 day cycle each month. Yet this month, nothing! I've had two negative pregnancy tests. 
I've looked into stress causing hormonal imbalance. I've had the most stressful year of my life but things are better now than before, so I can't understand why it would be regular through major stress and now I'm just skipping a period when things are on the up. 
Does anyone have any experience with this?