Anyone else?

I have always have a pretty regular period as in it would come almost at the same time each month. Lately for some reason its been skipping it for some months for example in December and February I didn't get one and the last time I had it was in April and it is now June and nothing! I'm not pregnant I've taken many tests and they are negative and I'm not on birth control either. Has anyone ever gone through something like this or have any clue as to why it happens?