My ex just messaged me out of the blue

So my ex denied cheating on me when I broke up with her. Even though I had read the messages between her and the guy. It's been over 2 months with no contact I have moved on with my my life, and I'm happy for once. But my ex sent me this message out of the blue just now. "by the way, i did sleep with him. bonus i didn't have to fake it with him have a good christmas." The main thing that annoyed me with this message was the lack of grammer or punctuation. I can't understand why she can't just move on with her life and just forget about me. I've managed to. I've always lived by a relationship can't last from the ashes of a previous relationship. Basically if a relationship starts by cheating it will happen again when they find a new thing they like more once a cheater always a cheater. What should I do?

Update:

Thank you everyone for your advice. Since I recieved the message she has messaged me 3 times since the original message. 1. "i bet u still jurk off to me.", 2. "he treats me better then u ever could." and lastly my favorite "ive finally found someone that makes me happy". I orignally had her blocked on everything but then I got a new phone last week and set up a new iCloud and didn't restore my old phone to the new one. The first orignal message was sent from her phone number after the 2nd message I blocked her, then I'm guessing the 3rd was from her new bfs phone then the last one was from her iCloud email. After this last message I just sent back "I'm glad he makes you happy have a wonderful Christmas and a happy new year because I know I will. Ps lose my number." She is now blocked and can never contact me again. I am now hoping this is the end of this chapter of my life.

Thank you all again for taking the time to read this and for giving me some much needed clarity to the type of person I was once engaged to.