hi. i have the same problems. i know you've had blood taken, but did they check your thyroid levels? because those can REALLY mess up your periods. Even if the level was only off by .1, it could make a huge difference.
Hey, This is exactly what happened to me too!!!
Only I had my 2nd period 7 months later!! ANd everytime it used to last for 12 days or more.
I consulted a doctor and she gave me some kind of pill. It worked for 5 months, I had blood test and urine test. I had ultrasonography too and the report said that I had 'a tiny cervical cyst', otherwise everything's normal.
However the problem vanished when I started doing yoga. But my period becomes irregular again if I stop doing Yoga.