In as much as you have gained appetite and feel tired, it is less likely that you coulod be pregnant since your unprotected intercourse
was just two days following your menses. That doesn't correspond to a fertile period unless you are not sure of your menses date and thus its less likely that you are pregnant. More so, you did do a pregnancy
test which as you said came back negative. However, the results of a pregnancy test are credible only when done after missing a period.
Black/brown discharges do not appear normal and in most cases indicate an infection. It is more likely that you have an infection that is causing you to feel sick and tired and to b having the black brown discharges.
It will be better to see a doctor for evaluation and vaginal smear tests and you will have to redo the pregnancy test jus to be sure.
Best regards and hope this helps you.