The alarming thing here is not that she's being quieter than normal for whatever reason. The alarming thing is that you think she's lying to you. You don't trust her to be honest with you.
If you can't trust her, it's time to reconsider the relationship. After all, it's not much of a relationship if you can't trust her to be honest with you about her feelings.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.