Why are my periods continually late?

i started my period almost 8 years ago, and as far as i kno they were always really regular. last year, october my period was about 3 days late. my november period was almost 3 weeks late. from december to march they became regular. within the last 3 months they have been 4-6 days late. in the course of this time i have had unprotected sex a few times (starting back in January). ive taken countless pregnancy tests all were negative. if im not pregnant wat could be the cause? im just confused about this. i have had sum changes in my life that have increased my stress levels.