It has also been mentioned that perhaps through the force she saw her mother when she was older. I don't know there are a lot of theories flying around about this.
maybe in a dream...but she didn't even know she could use the force until she found out Luke was her brother. I guess that's kind of obvious, though. ??? where am I going with this?
See the thing about those that are force sensitive is they can use the force without realising it and without even knowing they can, so she could have done it without realising it. A vision of the past through the force as a child.