I broke up with this one 6 months ago because she lied to me (no cheating on me though). I can say she really loved me, she was destroyed for months after the breakup. I was unhappy with seeing her so sad.
We met a couple of times last month to see if we can be friends. Things got sexual again pretty quickly and now we're probably going to give it another try in a relationship. But there are some things that seems weird to me.
First, there's a guy, our mutual friend she knows for about a year. Before me and her were an official couple, he was hitting on her at one party. He stopped (at least in front of me) when I dragged her away and later told him she was with me (turns out he asked her how are things going with me beforehand). Then they became a sort of friends.
This guy surely knows about pickup and although he's an ass and everyone who knows about pickup gotta see how incredibly fake he is, the girls fall for it. And to give him some credit, that's what counts. A lot of girls are good friends with him, but I expect not many of them to be more than that. Although his game is decent, I guess he failed to move things to the next level with most of them.
But he and my ex might be a different story. She talked to him about our breakup a lot (which might be a LJBF sign). She posts songs and internal jokes on his facebook wall from time to time. When they are together, there's usually some hugging involved.
Turns out they (with another 8 or 10 friends) were on a trip last month. There are some pictures of him hugging her, him touching her butt, and so on. The picture comments from the photographer jokingly assume the two have something going on.
She knows I don't like him much but she always said he was just a friend. A bunch of people (including me, her and him) were together for the last 5 days and me and her had some conversation. Turns out she would like to be back in the relationship and so do I. In the 5 days, we always slept together, held hands, hugged each other, caressed, had sex every day, just the stuff people like to do.

Now back to the part that concerns me:
For example when I told her to go out to so we could do some snowfighting, she turned me down stating it's too cold. Half an hour later he proposed to go out have some fun in the snow to which she agreed. It is fair to mention there was like 8 of us going while my "proposition" included just me and her. I also see she often looks at him. They keep hugging, she is often in his proximity, they share internal jokes, etc.
I confronted her with that saying that I see this as an obstacle, given my experience and I know that friends do behave a bit differently (I did not argue, did not give her any examples, just pointed out their relationship in general). She really looked unhappy and tried to get me to say what EXACTLY worries me so she can improve that. I did not tell her as I feel that would help nothing. She again stated they were nothing more than friends and that if he liked her that way, he would have tried something already (her exact words). She even said she talked to him about this and they laughed it off. She says I live in paranoia about him hitting on her (I know he at least WAS hitting on her plus I guess she can sense it).
On the other hand, she stayed in my company for most of the time, we had the new year french kiss, some bonding, she kept saying I am really good to her, etc.
I know the guy hates my guts. I hate his. I guess he respected my request back in the days when I told him I am getting on well with her and that's some fair play, but he's got agenda anyway....just so I cannot see it. Or maybe I am really paranoid. I don't know what the hell is going on.
Is he really just a friend? That worries me the most. She sometimes used to try to make me jealous.....an she failed. Now I really am jealous because for the first time I feel I might have a reason. You guys might save me a lot of bad nights.

The girl and i click together very well, but I am not going to invest myself unless I know she prefers me.