Sudden irregular periods

I've always had a 7 day period, very heavy with a 28 day cycle. Regular to a T. Even on birth control. All of a sudden the last few months I have started spotting 2 days before my period, then I have 2 heavy days like normal and at most 2 more days of my period. It's so short it feels weird. I'm wondering why it is suddenly short. I did not start or stop birth control recently. My doctors thought it may be thyroid related but after several months of testing they have ruled that out. Any idea why I'm suddenly irregular?