is my acne really hormonal?

Hi I am now 32 years old and have been suffering with acne for many years (since age 15).  The only time that I have ever been clear was during my pregnancy at age 27.  I was clear during the entire pregnancy and also during breastfeeding.  And I mean not one pimple which is unheard of for me!  Well it seems obvious that it must be a hormone thing, but birth control (Yasmin, Tri Cyclen, Diane 35) pills don't do much and all of my hormone tests (blood) came back normal.  How is this possible?  Could it be something else that I'm overlooking or could it still be hormonal but I just haven't found the right treatment?