What are the possible causes of a very long menstrual cycle?

Patient's question:

Detailed medical condition and consultation purpose: Her menstrual cycle has always been irregular (what's going on?), but this time she says it hasn't come for 3 months. Recently, she has also started experiencing nausea, but a pregnancy test came back negative! What could be the reason? Are there other ways to determine if she is pregnant? Based on these symptoms, can we conclude that she is definitely pregnant? I want to ask what symptoms are associated with pregnancy at 3 months? Are they more severe than these? I hope to get guidance and advice! Thank you
First follow-up question: So, does that mean we can be certain she is not pregnant? Could it be an issue with the pregnancy test?
I'm not very keen on going to the hospital!

Doctor's answer:

Hello: According to your description, if you have been amenorrheic for 3 months, you should have already detected pregnancy with a pregnancy test. You can now rule out pregnancy. It is recommended that you visit the hospital promptly to investigate the cause of the amenorrhea so that appropriate treatment can be administered. At the same time, please pay attention to the following:
1. Maintain a positive mood and avoid mental stress and emotional fluctuations. Some individuals may experience lower abdominal distension, lower back pain, breast tenderness, mild diarrhea, fatigue, drowsiness, emotional instability, irritability, or tendency to depression during menstruation, all of which are normal and should not cause undue anxiety.
2. Pay attention to hygiene and prevent infections. Maintain cleanliness of the external genitalia. Abstain from sexual intercourse during menstruation. Keep warm and avoid cold exposure. Avoid overexertion. Those with heavy menstrual flow should avoid consuming brown sugar.
3. Underwear should be soft, made of cotton, and have good ventilation and breathability. Wash and change underwear frequently, and ensure that the exchanged underwear is dried in the sun.
4. Avoid consuming cold, sour, spicy, or other irritating foods. Drink plenty of warm water to maintain regular bowel movements. Individuals with blood heat should eat more fresh fruits and vegetables before menstruation and avoid foods that stimulate fire, such as scallions, garlic, chives, and ginger. Those with qi and blood deficiency should increase their nutritional intake regularly, such as milk, eggs, soy milk, pork liver, spinach, pork, chicken, and mutton, while avoiding cold fruits and vegetables.