I'm really upset at my mom for taking money from me. Did I overreact?
Well, that depends.  Are you still mad at her?
No, I talked to her and let my anger out
That's understandable, people shouldn't be taking your money. You should tell her that it's wrong.
Some people only think it's wrong when they get caught.
It's good that you were able to talk to your mom about this when it happened, but if you think it might happen again, maybe you should have another talk.  Let her know that she needs to be more respectful of boundaries.