I think my girlfriend is up to something but I don't know what.
Why do you think she's up to something?
She's all kinds of weird to me lately.
You shouldn't think she's done something wrong before actually confirming it. She might just be going through something that she has to deal with. Just try to be there for her and if you do find something wrong then you have something to worry about.
But I don't like her weird behavior. She always hides her phone from me.
Her phone is her privacy. She doesn't owe you to look through her phone. If you can't trust her then you should break up with her.