Thanks for query to HCM.
As you described in query this type of menses call as irregular bleeding or dysfunctional bleeding.
However there are so many causes for this type o bleeding but few of them are most common .
1 Poly Cystic Ovarian Disease
( pcod or pcos) is most common reason of such type of bleeding in this age group ( 33 years ) . Diagnosis can be confirmed by ultrasound
of both ovaries .
2 The 2nd possibility may be any disease in uterus . Diagnosis can be confirmed by ultrasound of uterus . As you mentioned that there is dark colored blood so any tumor or fibroid
in uterus sometimes produces such type of blood .
imbalance is another most common cause in young female . Get in estrogen
, androgen , FSH /LH ratio. As hormonal imbalance also occurs in pcod .
In my opinion consult a gynecologist and get his opinion as physical examination . clinical history also play an important role to rule out any disease.