Hi!! Im Concerned Because I'm Having Very Dark Almost Black

Hi!! Im concerned because i'm having very dark almost black blood during my periods it's been happening since last year mid Sept. I did asked my doc??? And she said it was just acomulation of blood but i think it's been too long and this never happend before! I had tubal ligation 7 years ago and im healthy no problems at all im 30 yrs old and 5.4 143 lbs please help me im very concerned!!!!