Can I tell my mom that she's acting creepy?
Why would you say that to your mother?
She's been reading my journal.
It's good to be honest about how her invasion of privacy makes you feel. She was wrong to read your journal.
I need to vent so that I can calm down
It sounds like your mom breached your trust and that would definitely be upsetting. Letting her know that you are uncomfortable with her actions is admirable.