Having Dark Black Period, Swollen Breasts. Have Heavy Periods, Anemic. Possible Cause?

Period -Dark brown, almost black blood and swollen breasts.

Hope you can help?
Firstly, just want to say that I have always had extremely heavy periods for the last 20years. I have suffered from anemia in the past and believe I'm still slightly on the low side on being anemic -i just top it up by having iron supplements now and again.
Two months ago, i noticed my period was very light-just a few pinky spots. I noticed my right breast was very swollen, thought I was pregnant and did a pregnancy test which was negative.
4 weeks later my next cycle starts and now my period is very dark brown, almost black and not heavy. My breasts still swollen.
In the last 10 years i have had many large clots too, which i think is normal(?)
What does this all mean?.

Thanks for your help in advance.
SM