I cant get over my boyfriends past flings...Help!?

Im 22 years old and my bf is 27. We have a 10 month old and live together. I moved in with him more or less as soon as we got together, and fell pregnant after 3 months of being together. I really love him and we were smitten when we got together. He tells me he really loves me too, and we are both quite jealous of one another. Anyway, when we first got together we started talking about past ual partners and it didnt bother me at first. Until one girl that hed been with 3 weeks before sleeping with me, started doing things on facebook that really got my back up (the girl is his sisters friend). She kept writing little things on his sisters wall, and commentin on old pics, and settin her default picture of herself sitting in his (now mine) house!!! Anyways i went on her facebook to have a nose, and shes vile, she puts herself all over facebook, got no pride, and is such an attention seeker, and i just cant understand why he ever went with someone like her. He makes out like hes shy, and that he never really liked her, she was interested in him and it was just something to do, but im so different to her and it baffles me why he ever wnet with someone like her. I know it sounds bitchy, but honestly, it makes me think he likes slags. Well anyways just recently i have found out my bf lied about sleeping with her but has done stuff with her i,e . And this in my eyes is still just as bad, i was angry he lied to me, he said he just wanted to up his numbers but he as done that with her. but the thought of him doing that to her sickens me, because he makes out hes shy and wont make the first move but yet he did that to her? he said its because he was drunk etc etc and he thinks shes a wierdo. But i cant help think he did like her, and it makes me angry because he makes out like hes got loads of pride, outs me down about my exs, and yet i feel like he liked someone compltely the opposite to me. And with her doing things to get my back up doesnt help, i also believe his sister has done abit of stirring. I just dont know why i dislike her so much, but its really effecting my relationship. I just feel like i dont know my bf sometimes! I need peoples thoughts and advice! pls x