I have dark brown discharge its the 3rd day that its heavy. What does it mean? I'm also irregular so I really don't know when will I have my period.

Unpredictable. Unfortunately, if you have irregular periods, your bleeding patterns and amount will vary. Brown disharge is older blood, therefore this could be your period or an anovulatory bleed. This means that you have irregular bleeding because you don't ovulate every month. You should see a gyn to undergo a complete workup for irregular bleeding. Anovulatory cycles can put you at risk for cancer.