I'm not sure.. But it could be that your HCG levels are too low to be detected by the ua and blood test... It could mean other things too, so it would be best to speak to your doctor about it before you worry yourself sick. Good luck and HUGS to you!! 

I graduated with a girl who did the same thing!  She was  20 weeks when she found out she was pregnant!  She had taken 7 home pregannacy tests and they were all negative, but ultrasound obviously showed she was pregnant.  Her little girl is 3 years old and very healthy.  It was a weird situation, but everything was fine!  She also has a little girl that was born in January of this year and found out at 8 weeks that she was pregnant.  Her levels were through the roof!  I hope everything works out for you!

My mom had a friend who all her urine tests came back negative for pregnancy even when she was 7 mos along.  She went on to have a healthy baby as well.  It just may that you don't produce enough hormone in your urine.  The dr can always do a beta level check (2 blood tests about 2-3 apart) to see if your beta levels are going up.  Good luck