negative result i'm taking metformin for 3months

Hi,
Welcome to ehealthforum,
The light brown discharge after 5-7 days of finishing with periods that you describe could be indicative of probable ovulation bleeding. Insulin modifiers like Metformin are known to have positive effect on ovary for egg release. If you have been sexually active around ovulation time the chances of pregnancy are possible. Spotting or lighter bleeding around expected period time could also suggest pregnancy possibility. 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. Urine pregnancy test kits usually start showing positive results after about 20 days post-conception (roughly corresponding to about 1 week after your missed/altered periods). 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). Wait and watch for your periods. Look out for development of any other new symptoms if pregnancy is suspected. If periods have not started within 10 days of expected period dates, or if you have got abnormal periods, or if you suspect chances of pregnancy, 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). Be in regular monitoring and follow-up with your treating doctor/gynecologist and report any new/abnormal symptoms immediately. Drink plenty of water. Take adequate rest.
Take care.