My boyfriend and I had sex last night around 11pm, and he said that he believes he came a little inside me. I didn't felt anything inside me when he took it out. Now I am kind of worried not knowing if I can possibly be pregnant or not. Today, around 4pm I just have my period. Now my concern is how long can the sperms take for a woman to become pregnant? Can there still be a possibility for me to be pregnant after my period the following day?
Before I answer, please be aware that you were lucky -- the situation you're describing presents a high risk of both pregnancy and STI's. The risk of infection is always a concern, and one that should not be taken as lightly as it often is. You'll want to get that taken care of or use latex barriers ASAP.
If you are certain that the bleeding you're seeing is your period, it's unlikely you're pregnant; you simply got lucky and had sex at a time when fertilization was not possible. Sperm can survive in favorable conditions for around 5 days. Favorable conditions are when the pH of the vagina is more alkaline, provided by fertile cervical fluid around ovulation, and to a lesser extent, blood during menstruation. So, it is possible for the sperm to survive, but extremely unlikely for you to ovulate quick enough to present a viable risk. You'll want to take a pregnancy test after 14 days have passed, and giving your doctor's office a call to check if emergency contraception is necessary is a very good idea.
While you're making that call, please schedule an appointment to get in to discuss your contraceptive options and get an STI screening. We do not advocate use of the withdrawal "method" for the exact reason you stated -- it fails so easily because most guys cannot pull out in time. As if THAT weren't enough, pre-ejaculate can contain sperm that may cause a pregnancy, as well as the same concentration of STI pathogens as semen. Our own founder was the result of perfect-use withdrawal! The failure rate is a whopping 28 per 100 pregnancies per year, and is often toted as higher for adolescents who often lack the necessary knowledge of their sexual response and self-control.
The male condom is an affordable, highly accessible, cheap method available to all -- only 2 in 100 women get pregnant per year with perfect use. Hormonal methods obtained from your doctor, should you desire one, remain the most effective forms of birth control available to women, with fewer than 1 pregnancy per 100 with perfect or typical use, depending on the method. Please do this for yourself, and impress upon your partner the need to stop taking these risks.