What Are The Side Effects Of Mirena?

I have a merina put on more than 3 years ago and never have any period since then.recently i have it removed on november 23/2010 and started having my first period on december 17/2010 and it lasted for 5 days..this month january 20/2010 and i still haven't got my period yet, have took 2 pregnancy test and was negative what could be wrong?also my breast are very swollen and pain at the moment could it be a sign of pregnancy?