First let me debunk the information you found. It sounds like you read chat rooms and bulletin boards and not the medical information. Many people spread this myth because they don't know any better and it seems to be a pretty good case against sexual play. I think the thought is in many cases that it is better to scare you into not doing what you did rather than to educate you and keep you safe that way.
Pre-ejaculate or pre-cum does not contain sperm. It comes from different glands and a different place in that male genital track. It does not have anything to do with ejaculation but with arousal. It does not even pass through the sperm.
The only way it can contain sperm is if he ejaculated recently and did not pee between then and now. The pre-ejaculate's role is to clean out the man's urethra from left over pee and make the ph sperm friendly.
It takes more than a sperm or two to get you pregnant. A man ejaculating less than 20 million live and healthy sperms inside your vagina is considered infertile. There is no way that pre-ejaculate can contain that many sperm, even in the most ideal circumstances - not even close according to research. Most sperm was found in low quantities and barely alive in men's urethra after ejaculation by researchers.
Sperm does not have legs. Its got to wiggle its way to the ovaries. Coming from outside the vagina is a huge task for a sperm and if that happens, it will be a miracle. As soon as sperm is exposed to the air, it starts to die. For you to get pregnant, its got to wiggle its way from outside into your vulva, find the vaginal entrance, wiggle its way in and up the vagina where arousal fluid wants to wash it back down, do a traverse to find the cervix's entrance, get through the cervix, up the uterus, through the fallopian tube where it has to wait for your ovulation to take place. After all this trouble it is probably halve dead and now its got to hope that you ovulate soon. You ovulate just once in a normal cycle and it takes only 15 minutes for that egg to be released. The egg itself will die pretty quickly after this if it does not meet a healthy sperm willing to court it and mate with it.
It is just not likely. That is why you do not hear about many virgin births (if any). When it happens, it is so rare that it will make the news. Of course that does not mean you should not take care and be careful. Not at all. Accidents happens easily and before you know it he ejaculated way up in your vagina or he got sperm on his fingers and he put it on your cervix. So use common sense.
After taking Plan B you will spot and bleed in odd ways and odd times. Plan B is a high dose oral contraceptive with just one aim - mess with your natural cycle. That is how it works, by changing your natural cycle in a radical way in a very short period of time. If it did not do that, you might as well be taking sugar pills.
Best of luck and be careful!!