So I'm in a relationship with this girl for about half a year. Things go really well, she told me many times she loves me, sometimes I told her I love her too in return, she invests a lot into our relationship, I think it's a healthy relationship. She works hard and saves all the money so we two could go on vacation in the summer.
But!
She broke up with her ex about 9 months ago. They were both going to study in different cities and wouldn't see each other. She also claims he started to be weak and she didn't love him anymore. She started dating me shortly after that and we became serious over the next months. In the first months, he called her phone 10-20 times a day, texted her like mad, etc. She basically ignored it all. Then, after 2-3 months, he calmed down. From a third party, I know about an accident when he tried to kiss her but she got mad at him.
These days, he texts or calls her once a couple of days, AFAIK. She didn't cut him off her life completely as sometimes they see each other when she's home in her town. I know he still wants her, he loves her and although he's got a new girlfriend, he would leave her and start again with my girl, if he had the chance. When they meet, I guess they just chat each other up, but what can I know.
The problem is - he's going to have a birthday party in about 3 weeks. He invited her, she said she would come if she has free time. She never told me about any of this, I know from a third party again.
Today, she messages me on facebook (it's nearly always her who starts the conversation), we chat up a little, then she mentions she's been studying all day and now she's going to go out for a while since there's a nice weather out there. I don't question her or anything. I call the "third party" and yes, she's going out with him. He wants her to help him choose some props for his birthday party.
So, he's in love with her all the time. She is not dumb, she has got to know that. He tried to escalate on her previously, she rejected him. He'll try again and again, I'm sure. Why is she going out with him? Why she didn't tell me she wants to attend his party? I'm sure she knows this is something I might want to know.
Now this kind of pisses me off but I'm unsure about what to do. Should I somehow tell her it's not okay for her to see him? Or just be straightforward and tell her it doesn't make me feel well? Any advice is greatly appreciated. Thanks.