You know, she might be mad, but truth is you can do whatever you want. You don't need her permission to sleep with whomever you want.
My real question is...do you sleep around for sexual pleasure or do you do it to find true love and it just so happens that things don't work out as planned? I am not a priest or anything nor am I going to spew some religious bullcrap to you, but maybe you need to think about things before you go around like that.
It's not a crime with what you did cause as you said it is your Ex and so you have really nothing to fear. If you did get caught and scorned for it, then maybe you need to do something differently...something more close to the heart rather than what is in your pants.
I am not going to judge you and if you think I am I apologies, but take it from me who has been abused by 2 Ex-GFS, you need to find someone who is willing to compromise rather than point the blame.
Your best course of action is to take it in stride. If you get caught, then all you need to do is explain and that's it. If she wants to be a friend, then she should understand. If she doesn't notice and you manage to get away with it...only more power to you I guess.