Period 18 days late..

My period is 18 days late and I’ve taken pregnancy tests but all negative. I’m having a lot of pain in my pelvic area, mostly on one side and back pain. I have some nausea here and there like I usually do on my period but still no period. What could be going on? I’m never late. My period usually starts the day before or day of. 18 days is ALOT for me! Also, I spotted a couple days before my period was due but only when I wiped.