What Causes Pregnancy Symptoms With Negative Tests?

My last period was from June 21, 2010 til June 25, 2010 and today is September 4, 2010. I have been pregnant twicw so i know the symptoms and i am having all the symptoms from my prior pregnancy but the urine test and blood test that my doctor gave me were negative. She had my thyroid checked out and it is perfectly fine. Please help me I don't know what to do or what to ask my doctor to do. I really would like to know whats going on with me.