20 days LATE

So I am trying to keep myself calm so that stress doesn't delay my period, BUT this is my issue: I was on Depo from February-august and was due for my next shot in November (gradually had little to no period at all). I couldn't get it bc of complications and had a period from the 7th-14th immediately after missing my injection. I did have unprocted sex (that I recall) on December 7th and 11th, but that wouldn't even line up with the estimated ovulation. Now I am expecting a period ... that is 20 days late. I took 4 PT and they're all negative. What could be the reason? (shown is the estimated ovulation calender for November for which I know for a fact I didn't have sex during the entire thing)