Irregular periods with severe pain, nausea, and diarrhoea despite a normal ultrasound can be due to hormonal imbalance or conditions like dysmenorrhea or endometriosis, and the thin build with anaemic appearance suggests you may also have iron deficiency or nutritional issues contributing to weakness. Treatment usually includes correcting anaemia with iron-rich foods or supplements, managing pain with safe medications prescribed by a doctor, and regulating cycles with hormonal therapy (like oral contraceptive pills) if needed; simple measures like regular meals, adequate nutrition, hydration, exercise, and stress control also help. Since symptoms are significant, it is important to consult a gynaecologist for blood tests (especially haemoglobin and thyroid levels) and personalised treatment rather than ignoring the problem.