In this instance, doctors would typically advise for you to repeat the pregnancy test 1 week later.
As long as you are sexually active (and especially if you had unprotected sex), there’s always a chance that you could be pregnant – pregnancy tests have a high false negative rate.
If the pregnancy test is done more than 2 weeks after the last time you had sex, and the test is repeated 1 week later, you are almost certainly NOT pregnant – assuming you performed the test correctly.
FYI: OTC pregnancy tests have a high rate of false negatives (telling you that you are not pregnant, when you are) because it can take a while for human chorionic gonadotropin (hCG) hormone (produced when you are pregnant) to reach measurable levels in your urine.
There are many causes of brown discharge (including STDs potentially, if accompanied with other symptoms like pain on peeing, bad smells etc), but the most common cause is old blood that took some time to be expelled.
PCOS exacerbates the likelihood of brown-looking discharge (old blood), as your periods are irregular to begin with.
Do follow up with a doctor/gynae for your PCOS, as there are effective treatment methods to address the symptoms of your PCOS, such as irregular periods, acne, excess hair, and elevated blood sugar.
Cheers
