buy another test and do it myself or treat the back pain?

Hi,
Welcome to ehealthforum,
The symptoms of backpain and nausea could also be suggestive of possibility of pregnancy. Sometimes urine pregnancy tests can keep showing you false negative results during early pregnancy as the kits are calibrated ones, so unless the hormone levels in urine reach a certain level, they do not show positive results. If pregnancy is suspected, look out for development of any new symptoms suggestive of pregnancy and consider visiting your 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 you are not successful with your pregnancy plans, you can also consider going ahead with getting infertility work-up done for you and your partner to see if there are any underlying causes for the infertility/inability to conceive. Treatment as needed (based on examination and investigations results) would be started to help you get pregnant. Be in regular monitoring and follow-up with your treating doctor/gynecologist and report any new/abnormal symptoms immediately. Have diet with foods rich in zinc content to ensure better maturation of egg and sperms. Monitor cervical mucus changes and the BBT (basal body temperature) charting to predict your ovulation and time your intercourse for having fruitful results. Have intercourse in positions that allow for deeper penetration for high deposit of semen. All the best.
Take care.