Thank you for sharing your question with EmpowHER.
Periods that last for three weeks sound difficult and uncomfortable, so let's see if we can help you better understand this.
Several things can cause irregular and long periods that last for weeks at a time instead of being a normal period lasting 4-7 days. Common causes of long periods include ovarian cysts, endometriosis, pelvic inflammatory disease, birth control. Sometimes being overweight can affect your period, but a lot of times irregularity is caused by excessive weight gain or weight loss in a shorter period of time.
Polycystic Ovarian Syndrome (PCOS) is another symptom that can cause very irregular periods. PCOS can cause weight gain and very irregular periods on both extremes of having long periods and having periods less often.
I recommend making an appointment to speak with your doctor about why your periods are lasting three weeks and you're having clots. Your doctor can provide an exam to determine the cause is and can help get you on a plan to regulate your period.
I hope this has helped.