What can cause a false positive pregnancy test?

I took a pregnancy test at home and it was positive, so I went to the doctor and that test also positive, I then had a ultra sound but nothing was found at all. So they did a blood test and it came back negative. How can this be?
my past history is I had a bone marrow transplant 3 years ago but now in full remission. I did however have full body radiation, and chemo. Is it possible for me to get pregnant now after going through all that???
I got kind of excited about the positive and then was let down hard when it was a negative just really confused. How would I go about trying to get pg again? Or is it not possible?
Thanks abentley28

[foxy1220]

People who take HCG shots will get a false positive.