No idea why my periods are always late?

Before getting in a relationship in November of 2013 my period was always regular from the getgo. Its always come every month on time. Now since being in a relationship (I lost my virginity to him) I am 20 btw... my period was regular for 2 months after I lost my virginity. It stopped for a month than came back and than stopped for about 7 months. I lost my mother in October 2013. My doctor said it could of been losing my mom but it doesn't make sense I didnt feel stressed or depressed about it. Im also over weight but like I said my period was regular since getting it in the 9th grade..just wanna know your opinions. Btw yes I have gotten checked out and everything is normal. Im not pregnant. I wish. But nope got that checked too. So please let me know what ya think, would be great thanks!!