Thank you for sharing your question with the EmpowHER community!
1) When you think you might be pregnant, taking pregnancy tests as well as visiting a doctor for an ultrasound are great actions to take. Since your pregnancy tests and your ultrasound were all negative, I think that rules out the chance that you are pregnant. Those tests combined are usually very accurate.
2) Ultrasounds are typically very accurate when used to detect pregnancies. Usually they can detect a pregnancy with 25-30 days of becoming pregnant. Home pregnancy tests can also be very accurate. They measure hCG, and some tests can detect it as early as 4 days before your next period.
3) Late periods can happen due to a variety of things. You are right that stress can cause later periods. Some other causes of late periods include changes in diet, working out more often, illness, travel, and some birth controls.
4) The most common causes of fatty liver disease include sudden weight loss, malnutrition, certain medicines, autoimmune disease, obesity, a family member who also had fatty liver disease, and excessive alcohol intake. Did your doctor tell you that you have fatty liver disease? There can be a connection between fatty liver disease and amenorrhea. When the liver doesn't function correctly, it can lead to amenorrhea. And when there is extra fat on the liver, it does not work as well as usual.
5) There are some different ways to induce your period. Some ways to do this include skipping ahead to a new birth control pill pack, reducing your stress, sexual intercourse, higher intake of Vitamin C, and the herbs parsley and ginger (can consume through parsley or ginger tea).
6) I would wait at least another 5-7 days. Remember that it is common for periods to be late sometimes. Implantation of a fertilized egg does not happen until 6-12 days after ovulating. Usually pregnancy tests are more accurate 10-12 days after ovulating.
If you are not planning on getting pregnant, it is important to use a form of protection or birth control when having sex.
I hope this helped! Let me know if you have further questions.