I am impressed by how calm you are OP. Really impressed.
Make sure to have a conversation with the girl at some point, but do not do it through text. Try to keep yourself calm and listen to what she has to say. If she starts throwing blame on you, don't even try fighting back because she is simply not worth it.
If she tries to apologize, and you feel like it is an earnest apology, or if there is some sort of background to the situation, use your judgement. I certainly wouldn't tolerate something like this, but you guys have history together, I am sure.
Be aware that it isn't unusual for someone that cheats to cheat again. Use that knowledge and whatever she says to you and weigh your options. Is she worth all this trouble?