Hi Jamie thank you for your question.
I will start by saying that it is very normal for women to have irregular periods for the first months and, sometimes, the first years of their period. Your bodies` level of hormones is changing during adolescence, which can cause the length of the period, the time between periods, and the heaviness of the period to vary. Some women continue to have irregular periods until menopause (the time when menstruation stops). Therefore, this may be why you are not having your period yet and why it has not always been on the same day. However, there are many other factors that affect the period. If you have changed your exercise and/or dietary habits, or if you are experiencing different levels of stress, then your period can be affected. But, as you rightly point out, pregnancy is obviously another thing that can cause a delay in having your period.
Being that you and your boyfriend had sex 2 months ago and you have since had your period, I am not really concerned about pregnancy. However, I put the word period quotes, because sometimes women are pregnant and they may have a short time of bleeding after the pregnancy. If you had your period for its’ regular length and strength, then there is pretty much no chance you are pregnant (assuming you have not had sex since that 1 incident). If, instead, the bleeding was just some spotting or a bit more, then it is a possibility that you are pregnant. Bleeding during pregnancy may be a sign of loss of the fetus or it may be completely benign. Some women do not initially experience signs of pregnancy, i.e. morning sickness.
Seven seconds of sex may be sufficient to become pregnant. While it’s much less likely than if you had sex for a longer period of time and ejaculation, it can still happen. Even if your boyfriend did not have an orgasm, you can still become pregnant from the pre-cum (the fluid that leaks out of the penis prior to ejaculation). The best protection from pregnancy as well as from sexually-transmitted infections and HIV is a condom, which you should be using if you do not want to become pregnant AND to prevent catching infections (though you can still catch herpes and genital warts despite using a condom). There are other forms of birth control, i.e. oral contraceptives, the diaphragm, the ring, etc. However, these will NOT protect you from sexually-transmitted infections and HIV.
If your last period was very short and not like your normal periods AND you do not have your period this month, I would suggest going to see a physician. The physician will help you figure out what`s going on and can make suggestions for future sexual activity. As you are over 14, your visit is completely confidential.
Whether or not you go to see a physician, please make use of a condom and/or other forms of birth control in the future, to protect yourself and your partner.
Good luck and take care of yourself.
Lorin for Alterheros