very confused...

I posted about open relationships a few days ago. My friend/lover called me on the weekend to come over to help him pack and hang out. Before I spoke to him I thought I had it all sorted out in my head, just to let things go and see what happens. But even hearing his voice on the phone made me furious and very sad. I made an excuse not to come over as I thought I should calm down before we talk and figure out what is going on with me.
My anger is coming from the fact that he never openly told me from the start that he was in an open relationship with someone else. Like I said in the earlier post, I knew he was in some kind of relationship because of his facebook page. I noticed this a few weeks after we hooked up. At the time, I didn't seem to care, didn't ask him, and just went with the flow. A few months later he finally mentioned something about open relationships in a general sense, but didn't say specifically that he was in one or with whom. Again, I didn't ask. maybe I thought it's better I don't know or that I could be cool with it. Now I am thinking back and wishing he had told me from the start so i could decide if I wanted to get involved in this kind of relationship. One that is either casual or part of an open relationship with someone else. Not sure it would have made any difference, but at least I would have been aware of the fact that this would not lead to something more monogomous.
Do I have a right to be angry about this? Or am I just being petty?
He knew I just came out of an intense emotional breakup with my ex as well and was/is in a vulnerable state. But I do take responsibility for my own actions. I could have ended it or brought it up the minute I knew he was already involved with someone else. At the time it didn't seem to matter because it was so much fun and free.
I am not having fun anymore and feel hurt, angry, and sad.

Just thought i'd like to get some outside input before i talk to my friend/lover.
On one hand I think I have some right to be angry and hurt, but on the other hand I feel like a crazy unstable emotional woman! Please help!