Why is she sending me message on FB using my ex bf FB account?

I broke up with my 5yr bf 4 weeks ago because he cheated. So the third party then was now his gf. I didn't unfriend him in FB but I'm not communicating with him since the break up, not even a single text. So they flaunted their pics together, and me keeping myself busy and trying to move on and love myslef.... show more I broke up with my 5yr bf 4 weeks ago because he cheated. So the third party then was now his gf. I didn't unfriend him in FB but I'm not communicating with him since the break up, not even a single text. So they flaunted their pics together, and me keeping myself busy and trying to move on and love myslef. Going out with my friends, and his friends too bec. we also shared common friends. Recently his youngest sister ad me on FB and sharing things, missing me and being sad about the breakup etc...

Then boom, I got a message from my ex bf. But not, it was his gf using his FB. She said she had no idea why I was still communicating with his sister and friends etc and that I should leave him alone because he was happy and she wished me to understand and be mature about it.

I'm so pissed at first. Because im not bothering my ex ever since, she should've unfriend me and blocked me on FB. I think she should be the one to be mature and understand because in the first place, she should've not dated/flirted with my guy she know who has a gf (us). Do you think it was really my ex new gf or it was just my ex bf? What's the best reply to that message? Knowing that whoever it is, may ex bf will still read it because it's his FB account.

Thanks!
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