I'm upset at my mom
Why are you upset with your mom?
She said that she didn't have time to talk with me about something important.
That's tough, because in those situations you expect your parent to be there for you. It's okay to be upset with her for that, I'd talk to her about how that made you feel.
I might talk to her later.
I am sure your mom was just busy and absent mindedly forgot that you wanted to talk with her. Approach her later and she will show you the concern you needed.