I also thought about the Leia thing. Ben and I had an intelligent geek conversation about it. He pointed out that because Leia has the ability to use the Force she could probably feel her mother while in the womb. That's just an explation..maybe someone should e-mail it to Lucas to cover up the plot hole.
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.