Hi I posted this in other forums but just realli need some advice...Plz help:(

hey i'm 17 and just recently started to have sex with my bf. It felt right becuz we realli love one another, have been together for a long time, and can see our futures 2gether...Even his mom talks about "when we get married" well newayz...We just had unprotected sex and it got me worried cuz it was spontaneous...I mean he withdrew from me when he ejaculated but i'm still worried becuz of pre-ejaculation...I went to buy spermicidal foam afterwards just in case but I kno its not effective unless u use it be4. Newayz i'm realli scared...We made a pack to take extreme precautions next time but i'm waiting for my period this time...And just to make sure...If I get my period that means i'm not pregnant for sure right? Plus my period is weird...I have it each month but at different time...Like sometimes begging, middle or even end of the month...How does the cycle work?...Like when they talk about like 8 day or 14 day...*sigh* i'm realli worried and just need some advice and a person to talk to...I can talk to my bf but I also need some1 who's in the same position or have been...