Most Helpful Guy
Firstly, you're not pregnant.
1) Sperm on the outside of the vagina without the protective semenal fluid is highly unlikely to survive the journey through the hostile environment that is the uterus.
2) 2 days before your period is pretty much in the ideal window for when you are least likely to be fertile. (See ovulation charts.)
3) You took a pregnancy test on the 15th, plenty of time for your body to be producing the hormones required to make for a positive test.
Most Helpful Girl
You're a bit paranoid based on some astronomical odds
Precum is less than like 0.5% sperm.
Factor in the "prime pregnancy success rate" of 10% during ovulation with a big load, you're looking at a 0.05% chance of being pregnant under OPTIMAL conditions. This still isn't factoring in the oral transfer aspect.
Hopefully these numbers put the mind at ease.
I'm Asian, it's math. Insert joke.