precum cause pregnancy ?

hey everyone...please dont close this just cuz u see a long paragraph im really worried ....i need someone who really knows their stuff to please answer this im really worried...i had sex with my girlfriend around 3 days back....when we were having foreplay..she sat on my penis (without a condom on)...several times...but she sat on it then i took it out then maybe 3-5minutes later again this happened maybe 10-20 times however there was only 1 up and down action between each penetration and then we stopped...she came SEVERAL times and A LOT at one point (i cant really remember if it was after i came or before) but i noticed there was a goey substance on the tip of my penis...so my question is....i knwo the whole precum thing exists...how high chances are of getting pregnant off of this?...we had a worry before maybe 1 motnh back we had unprotected sex i came outside etc and she took 4 birthcontrol pills (she has heard this kills everything inside her) just as prevention and well nothing happened..she just had an irregular period 1 time...she proceeded to the same thing this time...however this time she felt bad the day after she took them and vomitted...is this just a sideeffect or something i should seriously worry about?..any tips as to what we could do if theres any chances for her to be pregnant??any advice or tip could be great im seriously not ready for anything of this sort im 18 :S...thanks for help