Who is a friend?

Something happened to me today. I am still trying to decide if it is grave or just something I should get over already.
A year ago I used to date this boy, I take responsibility for the fact that even if I like to blame my friends a lot that their pressure pushed me to date him, I still liked him in my own way. Maybe on a normal good day, it wasn’t enough for me to date him but I still did. We broke up a month after, well in my mind we did but officially it was until three months later because I was home for the summer holidays and he was in school taking summer classes.
I though that was water under the bridge until when a close male friend of mine(who is also his friend) asked me if he(my ex ) was dating my friend( lets call her lady).
I answered in the negative because to the best of my knowledge they weren’t dating and I felt if either of them liked each other I would have known or the would have told me namely lady would have told me because she is my friend.

Well turns out… it is true! They are indeed dating each other!
Am happy for them truly I am but it turns out again that another of my friend who isn’t in the same place as us knows but she didn’t want to let me know because she didn’t know what my reaction would be like. That has me thinking, why would I react another way, I mean that’s why he’s ex, no longer in my life… no more feelings… but I think they were thinking to the contrary.

Well as the story goes, lady didn’t tell me about them, my ex didn’t either (even though we are quite friendly with each other and he tells me about other girls).
Then this got me thinking that maybe what they did, in their eyes, its wrong?

Am not too sure what is wrong or right here because in my honest opinion, I don’t care who he goes out with, but my other friends are telling me its wrong for her to go out with him, like its an unspoken rule for girls, never date your friend’s exs’ or crushes.

I don’t really mind but the fact that they have gone to such lengths to hide it from me is beginning to make alarm bells go off in my head and lately lady hasn’t been hanging out with me. Never a day went by where she didn’t come to my room or call me. Now, I hardly even get her on the phone when I call her, I have called her twice to come visit her and she always tells me she’s away and insist she would come see me instead. Well now that I know she was indeed hiding something.
And my ex, well lately too… he’s been kind of cold towards me… just even barely being civil to each other... I didn’t want to make too a big deal about it but now that I know this, should I?

Everyone tells me to act like I don’t know until they tell me themselves... but is that the right thing to do? Do I even have the right to expect them to tell me anything considering that they owe me no explanations or do they?

Am quite confused and don’t want to act in a manner that would jeopardize my relationship with either of them, am quite clear in my feelings regarding both of them…
My ex: I couldn’t be bothered really. I wish him the best.
For my friend: am kind of disappointed she didn’t tell me because she confides in me about other stuff… so it’s this hidey attitude the have about it that has got me questioning…
WHAT DO I DO?