This is a lot of fuss for oral hsv1.
First, she didn't get oral hsv1 from a makeup brush. Also, with everything happening today, a professional makeup artist isn't going to be sharing any kind of brushes that could possibly spread covid. Sharing drinks, lipsticks, makeup, etc., are just really inefficient ways of transmitting herpes.
So, did she have it prior to meeting you? Maybe. Probably. Globally, 3.7 BILLION people under 50 do - 67% of them. 90% of people who have it never get symptoms, so most don't know they have it.
If she didn't, she could have gotten it from a kiss from a parent, a co-worker - anyone with a cold sore, or even without a cold sore. This is not evidence that she cheated.
Her statement that she feared she was having a herpes outbreak sounds like she knows what it feels like, and what was coming. She didn't say, "I'm having weird symptoms and I don't know what it is." Pure speculation on my part, of course. Did she test positive on a blood test? Did she get sores? What happened?
Did she know she had it? I can't tell you that. If she did, it's possible that her doctor told her it wasn't necessary to disclose that - many, many doctors don't think it's necessary to share that, even though we know it's transmissible via kissing and oral sex.
In any case, now to know if you have it. You can get a type specific IgG blood test. You say that you've been tested for herpes before - have you asked for that specifically? Many times, people think they have been tested for herpes when they ask for testing for "everything", and herpes isn't included.
Also, the hsv1 IgG test misses 30% of infections - a test flaw - so you may already have it and not know it.
No one wants herpes, and should take basic precautions to avoid it, but this seems like a lot of fuss over a really common virus, though I understand the "did she cheat or lie" part.