I missed my period last month but I’ve never had sex because I’m 15

Hello Anon

Thank you for writing!

We can rule pregnancy out.

There are many reasons for a delayed period or a change in cycles that is not connected to pregnancy. Some of these may be a factor in your life, others may not but I will give you a run down of many reasons.
Stress
When you are under stress, your brain may go into alert mode, which can change your hormone levels, as well as affecting when you ovulate.
Ovulation is the time when your ovaries release a mature egg that is ready to be fertilized. If the egg is not fertilized, and does not implant in the wall of your uterus, your hormone levels change, and your uterus cleans itself out to get ready for a new cycle for the next month. That cleansing process is your period.
If you are stressed, you may ovulate later than normal, or not at all one month, which can delay or cancel your period for that month.
Hormone issues
Hormones are chemical messengers sent out by your brain to regulate many different functions in your body, including your period. Hormones need to be in proper balance for your body to function correctly.

Polycystic ovary syndrome is a condition that causes your body to produce too much of the male hormone androgen. PCOS can cause cysts to form on your ovaries, which can prevent ovulation and keep you from having your period on time, or at all.
Problems with your thyroid can cause the gland to release too much or too little thyroid hormone. Thyroid imbalance can also affect when you have your period. In this case, medicine to help your body have the right amount of thyroid hormone can get your periods back to normal.

Weight issues
If your weight is either significantly lower than normal for your height, or if you are very overweight or obese, you may not have normal periods, as well.
Eating disorders, such as bulimia and anorexia, can also keep your periods from being regular. If you have an eating disorder, or are concerned about your weight, talk to your health care provider.
If you recently made a big change to your exercise routine so you are exercising more, that can make your period irregular. Endurance athletes often miss periods when they are in training.

Medical conditions
Some medical conditions can also affect your period. Diabetes that is poorly controlled can make your period irregular. Celiac disease, which causes inflammation in the intestine, can affect your nutrition and can also make your period late.
Other diseases such as irritable bowel syndrome, tuberculosis and liver disease can affect your period.
Medications
Some medications can make your period late or cause you to miss periods entirely. These include antidepressants, antipsychotics, oral corticosteroids and chemotherapy cancer treatments.
Traveling
A significant change in your routine, such as taking a big trip, can disrupt your body’s normal functions. Traveling can cause significant stress. It can also lead to changes in your exercise and diet routine. And if you travel very far, you may end up with jet lag as your body tries to catch up with changes in your sleep cycle. All of these things can result in irregular or late periods.

Talk to your health care provider to determine the reason. However, if this is just a once-off thing, consider it a hormonal blip. The body doesn't work like clockwork all the time, even if it's pretty consistent otherwise.
Best,
Susan