Is It Me Or Him?

I have been dating by partner for 7 months, and we are very much in love. He is quite a social person, and has lots of friends who are girls. There was a particular girl who he was friends with, and they had a short fling before we had met. From what he has told me, she had alot stronger feelings for him, and he didnt feel the same as her. At one point she left her boyfriend at the time for the chance to be with him. I met her for the first time a few weeks ago, when we invited her and a few other friends over for social drinks at our house (i have recently moved in and trying to make myself feel comfortable). She seemed nice, and we got along well, and I had no issue\'s with the previous attraction. During the night my partner took me aside and said she has tried to kiss him, and got too close physically. Not wanting to spoil the night, I said that he did the right thing by telling her no and we would talk about it later. This girl had quite alot to drink by this point, and spent the rest of the night doing a good job of flirting with my partner with me in the same room. At first I was ok, but she got more flirty and I started to brood on the problem for the rest of the evening. By the time everyone had gone I was pretty furious that he hadnt asked her to leave to said that she was being inapproriate. I dont think I would be as angry if it had of happened elsewhere, but I felt it was very disrespectful of her to be doing that in our house and infront of me, and also of him for not putting his foot down. We argued, and he agreed that he would have little contact with her. In the following days and weeks I found out they had been texting, and he had lied to me each time I had asked had he spoken to her. This happened on several different occassions, and even though he had sworn to have minimal contact and not strike up a conversation with her, I found out there were several more texts and that they had talked on a train ride to work (working close by to eachother). I have trusted my partner previously, but now im not so sure. I have never really been a jelous girlfriend before, but now I feel like I have a reason too, I genuinely feel like im competing for him. Its also made me paranoid that when he goes out he\'s going to see her/call her. He has sworn that he loves me, and wont talk to her, but he has been dishonest a couple of times already.I feel like the only way to keep tabs that hes telling the truth is to check his phone, which I hate doing. Am I being paranoid or obsessive...??