Why could my period be 12 days late?

I am currently 12 days late since my last period was 40 days ago.  I have never been super regular but the previous 2 months I was on exactly 28 day cycle (I've been tracking since November).  My husband and I unwittingly had sex on my most fertile day in December (New Year's Eve) but I had a urine test at the doctor's office last week and it was negative... still no period yet though and I am starting to get concerned.  Around when I was supposed to get my period I did have some symptoms though: cramping and very emotional as well as back ache, some acne and an all around 'yucky' feeling.  What other reason could there be for such a late/missed period?

[Bettybooker]

I would go to Dollar Tree and buy a pregnancy test.  They're cheap!  You can buy several!  

Take the test, you may actually be pregnant.  

If not, you may have Polycystic Ovarian Syndrome.  There may be other possibilities, but this is one.

You might want to get your Vitamin D levels checked.  www.vitamindcouncil.org
I've found that when I take my Vitamin D3 (cholecalciferol), my periods are more regular and lighter and better all around.  I use D Drops (Carlson), under the tongue.
During winter, we don't get much Vitamin D, the sunshine vitamin.  And there are doctors who believe the RDA's are too low.