Jealousy? overreacting? what am I doing?

Last night we had an argument. My husband has a friend, they text each other every four- six months, they text for about an hour, and during that time I can't talk to him because he's all focus on texting her, she does a lot of debates so they are always debating about politics, one night he wanted to cuddle and watch a movie, so we did, but the whole time he was on his phone, he told me that he was talking with this brother, but he lied about it because he was talking to her... I read their texts... nothing bad about their conversation! later he told me (6 months later when I told him that I looked in his phone) that he lied to me because he knows that I don't like when he talks to her. I barely know this girl (she's about 17-18) yesterday I saw her name (he told me to answer his brother's text) I got so mad, he knows that I get very angry when he texts her and we always have an argument about it! I just don't understand why I get so mad every time that her is on his phone, I know I get jealous but honestly I don't have a reason to be jealous, I just can't control myself, he tells me that there is no reason for me to be jealous and I believe him, I know he talk to anyone that he wants tk talk, I just want to stop this (my reaction) but I don't know how.