Hi I'm a 18 year old female and I have not had my period for 4 months now and I am not sexually active so I am sure that I am not pregnant. My periods have been irregular for a year now. I even got a ultrasound for if I had a cyst, but it was clear.
Symptoms : Mood swings ( very often ), aggression, stressed, anxiety.

Irregular periods is very common especially in the beginning two years.

The reasons for it can be either common or something that needs to be treated seriously.
Common reasons are eating disorders, travelling, over-exercising, and stress, emotional illness.
You reduce your anxiety, aggresion, stress and maintain your proper diet. Do not worry much about period, this is one of the reason for irregular periods. You may go to the hospital for the check up.

Hi I'm a 18 year old female and I have not had my period for 4 months now and I am not sexually active so I am sure that I am not pregnant. My periods have been irregular for a year now. I even got a ultrasound for if I had a cyst, but it was clear.
Symptoms : Mood swings ( very often ), aggression, stressed, anxiety.

18 years old, just out of high school and into the adult world. Time of stress and anxiety as you said, which can cause irregularites in your cycle. Talk to your doctor about this and he might put you on a hormonal birthcontrol pill. While having contraceptive properties these also help to regulate your cycle. If you're not too worried about it, or it doesn't pose a health risk then don't worry too much about. Just make sure to have regular doctor visits and you should be fine.