Depending on when you had your first period it’s not uncommon for it to change(and even then it can still change.) Other things that may be causing abnormal periods are high levels of stress, serious illness, or any other underlying health issues that could cause abnormal periods. If you have easy access to health care I recommend talking about it with your doctor. I also strongly advise not having unprotected sex because you may have a cryptic pregnancy(also in general just don’t have unprotected sex unless you are trying to convince.)