Period is three days late and had a negative test yesterday??

So my periods have always been like clockwork, usually 27 to 28 day cycles but the last two cycles have been off, last cycle was 25 days, this cycle, I’m currently on day 31 and still no sign of it. I definitely had some symptoms, I was moody, my boobs are sore, and I have a ton of discharge. What can cause periods to suddenly be off??? I have suspected endometriosis but It’s never impacted my cycles, their not heavy, always pretty moderate bleeding. I’m so confused. Can I still be pregnant, and get a negative test?