What Causes Brownish Discharge During Menstruation?

I'm 18 years old, and started my menstruation when I was 10 years old, hence it is relatively regular now. I had my period 3 weeks ago, which lasted 5-6 days. Less than two weeks later I started to bleed again, which lasted for 3-4 days. It has currently been 3 days since the last bleeding incident and I've started bleeding rather heavily again. The colour of the blood is dark, brown and is slightly mucus like. There are a lot of clumps and tissue too. I've been sexual active for 10 months or so which the same man. I am also a Thalassemia minor. Please tell me what's wrong with me and whether you are in need of any other information, this is really disrupting my university schedule and plans. Thank you!!