I've searched online and been really worried whether the things I've read are true. The tip of my boyfriend's penis had a contact with my vaginal opening, does it really cause pregnancy without pre-***? Thank you

A few here are doctors, many are nurses or related professions, many are neither.
It is not true that it causes pregnancy.
It is true that it CAN cause pregnancy.
The odds are extremely low when the tip is just at the vaginal opening, and even lower when no semen or pre-ejaculate is present. But who's looking when this is going on? How big is one drop, and one drop can contain a whole army of sperm?
All science knows is that women are constantly getting pregnant with very little contact. How many women get pregnant the first time they have sex? Lots. The rate of pregnancy using withdrawal is quite high, something like 25 - 29%. So before you get saddled with 18 years of motherhood, go get some good BC.

Hi, I'm 21, my last period was on April 10, 2012 that lasted for 5 days. I got delayed for over a month and a sticky blood/discharge came out last may 29, '12. Then the next day, I had an unprotected sex with my boyfriend and let him come inside me, thinking that it will help the blood to come out, but it didn't.. the next day, I took a pt and its negative. Then the same day, I took 1 nordette pill at 9:35am, then 4 pills at 9:35pm on the same day. After 24 hours, I took another 4 pills at 9:35pm, since then, I'm not sure if I have my period(because it has clots like usual, but the flow is really light.. almost like spotting, but it drips whenever I pee) or its implantation bleeding. I'm scared. How will I prevent pregnancy now?