She is very territorial and possessive sometimes. She has this very strong instincts that can sense if you are lying to her. So better yet tell her the truth.
She gets jealous because she wants your attention. She does not want to lose you. She understands that you have girl friends, and she is afraid that you might replace her with someone else.
You might call her paranoid sometimes, but it is all because she just cared too much for you. Her jealousy is her way of showing how you mean a lot to her.
She knows that at some point she is being unreasonable and sometimes you feel like being choked, but that is how she is and she is sorry for that. You better be scared if she suddenly stops doing that because she does not care anymore.