Any ideas?

My period was two days late, and it started this morning. I usually bleed heavily for the first 2 days and then light for 2 days. But I haven't even been filling regular tamps. And they are just covered in old brown blood. I was just wondering why that could be happening. I'm slightly cramping and everything, I just don't know why my flow is drastically lighter. And don't say to see a doctor. Because this is just a question of your opinion of why this may be happening.

Thanks!