One of my closest friends had a similar situation to this. Long story short, she found out she was pregnant when she was 6 months along. She had been testing the previous months but nothing had came out positive, and she had gained no weight, and had no "belly". She realized her stomach was kicking one night and finally went to the doctor who comfirmed obviously. The urine test at the office were also negative, the Doctor really had no explanation for my friend as to why this happened. She had her little girl 6 weeks early in December, she has serious health problems and spent a long time in the NICU. My friend is never going to be able to forgive herself for not making an appointment sooner.
I urge you, if there is any feeling that you might be pregnant please go to the doctor for a test. I know it is rare to not know for so many months but it does happen.
Not really an answer to your question, just something to think about. Good luck!