Late periods

So, I have been having some issues with my period these last two times. My last period was late by 2 months. I took several pregnancy tests and also went in to the doctor to get tested because I have never been that late before. I finally got my period about a week after going to the doctor. I am now 8 days late again and I don't understand why this is happening all of a sudden. I am sexually active so, there is always a possibility of pregnancy. Has anyone else experienced this after years of having normal periods?