I am a 15 year old female, 98lbs and 4'12, and had unprotected sex on December 20th, 2014. According to my calender my fertility was low. The sex only lasted one minute because we both decided to stop before we did anything even more stupid. I am in a commited relationship. My boyfriend did not ejaculate inside of me, or near me- he did not even ejaculate. However I have read that sometimes there are traces of sperm in precum. We are both positive that he did not have precum anywhere near his penis, however this poses a question. My period cycle usually lasts around 22-23 days. I had my period on December 4,5, and 6. I have a calender that tells me future period dates, and I was supposed to have my period again on Decemeber 28, 29, and 30. I did not. I have done vigorious research over this and I am honestly terrified. I first started my period when I was 12 years old, so I shouldn't still be having irregular periods, right? The only symptoms for pregnancy that I seem to have is frequent urination, and missed period (obviously). I am very worried and have learned my lesson. I would just like to know the chance of me being pregnant, and whether or not this is just the start of irregular periods? I've never really had any irregular periods before, so this is very odd. Thankyou.

The chances of you being pregnant are 50/50 simply because you had unprotected sex.

At 15 your periods are going to be funky, and they probably will for a few more years until your hormones have regulated.

The he only way to have a reasonably certain idea of when you ovulate is by tracking your basal body temperature for 6 months or longer. While the apps for periods and ovulation are getting better, they are still not accurate.

I appreciate that you think you are in a committed relationship, but at your age these relationships are rarely permanent. Then, when you throw a baby in the mix, they are never permanent and you will be left a single mother struggling for the rest of your life.

There is ALWAYS a chance of pregnancy with sex, it's no help to wonder what the odds might be (if you are pregnant, you are 100% pregnant), and they can't be calculated, because of countless variables.

Misconceptions in your question:
You can't predict when your fertility is low from a calender. Any woman can get pregnant any day of her cycle, and it can vary each month.
Yes, there can be traces of sperm leaking out at any time a penis is inside a vagina. The odds are just lower if there is no ejaculate.
Your period length can and probably will vary over your lifetime, and being regular for the last 3 years is not meaningful.
You are still well within the range. But you could skip a period this month. Holidays, change in diet, change in many things, and stress over the sex you had can all contribute. Random hormone fluctuations can too. Another part of this with too many variables.

Conclusion: Everything related to menstruation and pregnancy has far too many variables to calculate. Ditch prediction calendars (but do track your periods), get some birth control, and don't look for probabilities. It's too much like Russian Roulette.

It's easy for me to say don't worry, but I hope you can realize that it's too soon to worry. Do read up on using HPT's, but don't buy any yet.