This is way more common than you would think! That, however, does not mean that you should just live with it, ignore it or expect it to go away on it's own. If you are feeling this way for this long, it would probably be more accurately described as
Postpartum Depression than the
Baby Blues.
It is definitely not your fault, we can all just blame it on these dang hormones that were responsible for ALL of the thousands of emotions that we felt during pregnancy. We can never predict how we are going feel after giving birth, and we usually can't control it either.
Here is some info you may find helpful:
http://familydoctor.org/online/famdocen/home/women/pregnancy/ppd/general/379.html
Good Luck