Why my period could sudden become irregular?

 Since I got my first period in October 2010, it's always been pretty regular. It would come every 30-31 days like clockwork, except a couple months out of the year, but it would still be less than 7 days late. The last regular period I had was in April 2016. In May, my period came the day it was supposed to, on May 6, but it was pretty much nothing but brown discharge the entire time. June, my period came late, on the 11. Again, it was late in July; coming on the 15. Now, in August, it's 4-6 days late. I don't understand why my period has suddenly become irregular. I'm not sexually active (I'm a virgin with no experience), so it's impossible to be pregnancy and no STD's. I've not gained or lost weight, gotten on or off any new meds or ever taken any form of birth control. My eating habits aren't the best and I am overweight, so that's the only thing that comes to mind. However, I have been overweight since I started getting periods.