Your period is the breaking down of the placenta as implantation did not occur. The placenta is a lump of clotted blood that can be quite dark. If you are experiencing painful heavy periods, go see a doctor. If you are not experiencing other symptoms, then this is probably normal for you.
I doubt that anything is wrong with you, I'm not a doctor however I think it all depends on the state of your menstrual. Several factors can contribute to you having large bloodclots