Why is she a total bitch?

I have a boyfriend who is married to a woman who left him for another man. They had a handsome son whom at first was with my bf until the ex got him and had her parents take care of the child. Anyways, when my bf and I were staying together very well, the ex suddenly makes communication with my bf. she asks about stuffs that she can ask to someone else like her friends which leads me to think that she is just calling attention. Example she will ask where can she have her car repaired or if his brother still sells poke setr. Then adds the relatives of my bf on FB etc etc. Stuffs like that. But then inspite of all that I kept silent because i was confident enough that my bf is faithful to me and I give respect to the situation. My bf talks to her about the divorce. She said that it was her priority as soon as she finishes her board exams for medicine. After the exams, my bf called her to seek for update. She got really mad and puts her phone on voicemail. We tried sending her email to talk things out but then she ignored our mails. I dont understand why she is behaving that way! She had 2 relationships already aftr she got out of their marriage. Isnt she happy yet? Am I right to assume that she is a selfish bitch? Isnt it that by this time they should be civil with each other? Its been 6 long yrs since they parted. I need aome clarity. I want to understand. Thank u in advance for reading this and for taking time to help me understand my dilemma.