Dating someone who flirted your friend...

Well this seems to be a no problem to many people I've discussed with. But for me is a pain I can't seem to get away.

History goes like this:
1� This girl is introduced at same time to me and some friends.
2� On that same night she goes out dancing, and ends up connecting (they just danced but nothing else happened) with this friend of mine. I dont go out.
3� On the following 3 weeks due to some travel I spend many time with this girl. My friend is away.
4� In 6 months I flirted her in the msm, but couldn't conviced her to go out.
5� Later, and after we all 3 been together getting out in the group many times, I ask her to travel with me, which she accepted
6� A month later we are dating out each other.
7� After we've been dating for a month she tells me that at very beginning when we met, she went out with my friend for about a month.

She argues that she didn't knew about me (trying to date her) at the time and that she had a bad feeling about me due to some things she heard.
My friend (38) is a bit older than me (33). The girl is 31. Age is not a problem to me, but she told me she wasn't looking for a relationship at the time with him because of my friend messy relationships in the past.

It hurts because it seems that at some given point (the month after we came back from travelling) she decided to date this friend instead of me. It also hurts because they decided not to tell anyone.