Hi,
Welcome to the ehealthforum and I am glad to help you out. Ultrasound is the most specific test for pregnqancy and if it is coming negative for any fetus , then pregnancy is unlikely.
False positive pregnancy test is rare and is usually found in is found in chemical pregnancy (a pregnancy in which there is a positive pregnancy test and no other signs or symptoms), missed reaction time( when the test sits for too long), evaporation lines(faint line during UPT) and when a woman is taking any drugs which contain hCG. So my sincere advice would be to recheck the urine for pregnancy test by getting the test done by aimstrip test (highly accurate and measures even less amounts of hCG (upto 20mIU) or get a repeat quantitative blood test or transvaginal ultrasound done.
You should consult a gynecologist and discuss this with her. It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care.