My period is irregular and the last time it came on was july 8, 2017. two days ago i noticed thick dark brown discharge. my period is on now but is not red blood it's completely dark brown and there's no red blood at all and i'm having back pain

Irregular bleeding: This is may be normal but you should take a pregnancy test if it is even a remote possibility. Wait and see what happens. If symptoms are severe you will have to see your doctor.

Irregular period: Pregnancy and related conditions can cause irregular period with back pain . Irregular periods can be due to hormonal disturbance like polycystic ovaries, thyroid problems etc. Please do a pregnancy test and consult your doctor