Is all this in my mind?

I've been all over the internet trying to figure out what I am going through. I am hoping that someone here might have gone through this or knows someone that did. I am 27 years old a proud mommy of five children. After my last one was born I had my tubes tied that was 2 years ago. I have always had a very regular 28 day cycle. However I have not had my period now since January 5, 2009. At the beginning of March I realized that I had missed my last two periods, so I took a home pregnancy test. Accually I've taken a total of five tests from three different brands since then and had one come back positive.
I went to my doctor two days ago and he did a urine test. It came back negative. He refused to do a blood test or ultrasound saying he didn't believe it was neccessary at the moment, but wants me to come back in two weeks for another urine test. I asked him what else would cause me to miss my period for two cycles (three next week). He said a lot could, but he wanted to rule out pregnancy first.
I felt like he was contridicting himself. I don't think he wants to admit that the tubal ligation may not have taken or he did something wrong. I feel like I am going crazy. I have gained weight, up until two weeks ago I was sick everytime I ate, and I've had horrible heartburn. After having five kids I think I know what pregnancy feels like. The only difference is that all my other pregnancies showed up immediately on a home pregnancy test. What could be causing the tests to not show positive or is all this in my mind?