Brown big lumps of blood clot started 2 weeks before period. It lasted for almost a week. I'm not expecting to be pregnant. No intercourse. Why is that?

Many causes. The menstrual cycle is under numerous influences, both externally (e.g. Stress, weight changes, diet, etc...) and internally (e.g. Pituitary gland, ovaries, adrenal glands, etc...). Additionally, pathology like uterine fibroids and endometrial polyps can also intervene. Sounds like a thorough work-up is in order. Of course, a pregnancy is always a consideration. Get with your ob/gyn asap.