not that far along and the heart just isn't developed yet?

Hi,
Welcome to ehealthforum,
Early ultrasound scans can fail to detect gestational sac or fetal heart. It becomes clearer around 6-8 weeks when fetal heart beat can be seen distinctly. You can get false negative results with urine pregnancy tests done very early in pregnancy, as the kits are calibrated ones, so unless the hormone levels in urine reach a certain level, they do not show positive results. The test should always be done on early morning first urine sample to get accurate results (pregnancy hormone HCG is believed to be in highest concentration in this sample, and you should avoid lot of fluid intake and caffeinated beverages in the night before taking the test). Look out for development of any other new symptoms if pregnancy is suspected. If you have missed your periods, or have got abnormal periods, or if you suspect chances of pregnancy, or if the symptoms are persistent, consider visiting your doctor/gynecologist at the earliest for thorough examination (to confirm pregnancy related changes/signs in body) and blood/serum HCG level test (to confirm/rule out pregnancy accurately). If pregnancy gets confirmed, get repeat ultrasound scan to confirm gestational age and pregnancy location (intrauterine/ectopic) and viability. Be in regular monitoring and follow-up with your treating doctor/gynecologist and report any new/abnormal symptoms immediately.
Take care.