It sounds like the ETL does have it in for you, so I would say you need to be pretty meticulous in your response. Read the CA, and gather any documentation you can, including any and all texts and stuff like that. If you're about to go on vacation, that might be good (please someone correct me if I'm wrong!) because it will allow you to look over the documents and think about a response for HR and/or your STL when you get back. If the attendance issue is cleared up, bring whatever documentation you can (eg. I was late 3 times before the first week of December, but have been on time since then, here are my timecards, etc.). It's super easy in these type of situations to get rushed & defensive, and that's what the ETL is counting on, so your best bet is to remain calm, factual, unemotional and document, document, document.
Good luck!