Irregular periods?

So I have kept track of my past three cycles, they was 27, 26, 36, and this one should be 30 if this app is correct. My period was 8 days late but I started today. I thought I may be pregnate there for a few days but tested negative. My question is why would this be? Has anyone else ever experienced this? Anyone ever known it to mean something bad or get professional help for it? I have felt a little off for sometime...maybe hormonally off? Maybe thyroid? Thanks in advance for any help or advice and your time.