"She was in my stuff." Usually, this means that she was handling or looking at my things without my permission. "Carol was not only in my room, but she was also in my stuff!"

It means that she was looking through your belongings. Obviously she can't be trusted.

It depends on the context... she was in my stuff-she was going through my stuff / belongings. she was in my house, she was inside my house. she was in my private business, she was poking around my personal business. It is not commonly used slang. Hope this helps, let me know if you want to know anything else :)

could be lots of different things