On the surface I don’t see anything inherantly wrong about a person and their boss texting after hours.
However, it’s concerning why she felt the need to HIDE it. If it was innocent, then why couldn’t you be in on it? Even if it’s just in passing. You live(d) together, so if she is texting after hours, then the topic of what’s going on with that will come up sooner or later.
I will say that if people of the opposite sex are texting/communicating on a regular basis later on in the evening, 90% chance that at least one party is interested in the other. It’s highly likely, if it goes on for a significant amount of time, that both parties are interested in each other. Unless they are blood-related, I don’t think it really matters what the nature of their relationship is.
Ultimately, what beans said. If you can’t trust her for whatever reason (whether it be your insecurities or she actually isn’t worthy of being trusted), then you’ll either have to legitimately find that trust again (if it’s even possible) or move on. If she seemed like she had things to hide, then I’d be very curious about it, myself.
If what you’re saying is true and you’re not leaving out significant parts of the story, then I don’t think you’re wrong for being suspicious. Maybe kicking her out almost immediately was a little forward, it should at least be talked through in a calm way so that emotion won’t affect the outcome.