What did I do wrong?
You stated what you wanted to do but didn't have a strong enough boundary around it. She put on an extinction burst. You caved and she had her tantrum validated as a method to get her way, =intermittent reinforcement. Creates a "go to" method to having her needs met.
To stick to a boundary you need to preplan, as this is repetitive behavior. If this is her predictable reaction then you organize your life so the fall out is minimized on you and she is left to deal with whatever the consequences of her actions are.
The underlying trigger is insecurity. Validation works best if it is used first before you state something that may be a trigger rather than as an after the explosion fire extinquisher
Waverider