Paranoid

Ok so I had sex last Monday evening and he used a condom but it didn't split, I freaked out cause I thought it did and this is the first time a guy has came in me with a condom on so I was worried, I asked him if the condom split and he said that he promise it didn't because he would've saw when he pulled out. I went and got Plan B (Levonelle) the next day around 10:30 and took it then. Is it possible for me to be pregnant? Because I'm literally freaking out so badly. How effective is Plan B (Levonelle)?