Re: what to do?
I personally don't think you should be worried. The odds of getting pregnant this way are practically nil. Only, possibly, if you were near orgasm might there be any sperm in your precum, and if you produce little or not precum, I don't see how there could be sufficient sperm left inside her to cause a pregnancy. If, OTOH, you produce lots of precum, the odds might be a tad higher, but still pretty low.
It takes a decent glob of semen to fend off the female's defenses and make it all the way to an egg. Another risk factor, I should add, would be the number of days since her last period -- females are typically most fertile 14 days before their next period.
To conclude, the risk is not exactly zero, so, since you must think you are mature enough to be having sex, you must use that maturity to decide what to do now.