Thank you for reaching out to EmpowHER with your health question!
To start, if you and your boyfriend have decided to have sex at 15 years old and you are concerned about pregnancy, I highly recommend using some form of protection. Birth control and abstinence are really the only ways to reduce your chances of pregnancy. But I would not be having unprotected sex if you are concerned about pregnancy.
Any time you have unprotected sex, your chances of pregnancy increase. You mentioned several symptoms of pregnancy but it sounds like you also have an abnormal period so it's hard to tell. The only real way to tell if you are pregnant is by taking a pregnancy test. I recommend you take one of those to see.
Also, this article we wrote on EmpowHER talks about the pros and cons of different types of birth control. Take a look to learn more about your options: https://www.empowher.com/birth-control/content/which-method-birth-contro...