Period question please help!

Okay so I am pretty regular with my periods I mean the trackers are usually right on point give or take a couple days. My last period started on the 29th-Nov and ended on the 4th-Dec. my fertial days was the 5th-11th ovulating the 10th. I had unprotected sex with no pullout method on the 5th, 11th, 20th, 21st, and 22ed and my period started early on the 24th when it was due on the 29th and it was pretty heavy and painful on the first day and today is suppose to be the 4th but when I woke up the pad was clean and yesterday I barely soaked up the tampon I normally last 6-7 days. The last time my periods was this short was over a year ago when I was on BC. What could be reasons for this ?