Ex girl friend problems and just need to ask a question is that ok?

OK let me tell you what is going on. I and my ex broke up about 3 weeks ago. We dated for about 1 year. I thought she was cheating on my for about a month but could not prove anything. Tell about 2 weeks before we broke up. I found out she sent picture of her self to a girl she knows. But it was not nude pic it was just sexy pic. When I asked her about it and she told me a long time ago she had a bi think with her. What ever I don't care if she did or not. But to me that is cheating. Am I wrong on that? Because now I find out from her she started the BI think back up with her again. I am just very mad now. I want to get her back for what she did to me. I know that sounds mean but that last month was hell on earth for me. I found the girl that she is BI and she has a BF also. He does not know that his girl is BI. Also that she is not telling him also. So I am thinking about telling the dude because I think he has the right to know. I know the dude will break up with her because she will not have a three way with him. So it will end up just like me and my ex and my ex will get the bad end of the deal. Think I should do it or just stay mad at her. I am just so mad!!!