I don’t think you’re over reacting. You have given a punishment for a disobedience and a sub needs to remember the rules. She was honest enough to tell you the next morning and by this she trusted you to take appropriate action probably knowing full well she would suffer the consequences.
I know someone in a similar situation where the sub was completely honest with the Master by telling him she broke a rule. the Master punished her for disobeying but he also released her. On the surface that seems to be an over reaction to me. So the poor sub was mortified with herself for breaking a rule but then had to deal with being released basically for being honest. She was crushed. I think the Master in this instance was looking for a way out without being honest himself.
I think you both can build the trust again if you want to… it just takes commitment to the relationship and communication with each other.
Hope it all goes well for you.