My ex-girlfriend volunteered to be my friend. I replied when I talked to her, and then I stopped replying. Why?

To tell you the truth, I probably just want to see how you are doing and how she is doing. If not, she will be relieved. Don't brag, many girls are just curious about your life now, and then they are actually not interested in you. However, after all, she doesn't think you are as good as her, and I don't think she will say much to you. If she sees that you are much better than her, then you two may still have a chance, provided that you make up. Moreover, be sincere, not routine, it is really good, not pretending.