I remember a case from UM about a girl who lived with her grandmother and this lady came over and the grandmother sent the girl to the store and the lady was just staring at the girl.... turns out that the lady might have been the girls real mother. Anyone remember this?

http://unsolvedmysteries.wikia.com/wiki/The_Mother_of_Carla_Downing

I think this is the case you're looking for. The reenactment included the girl being introduced to her birth mother without knowing she was her mother and then finding out the truth years later.

Yes that's it! Thank you!

You're welcome! :) I remember this case being really touching and was happy to see that there was a reunion.