Did that really just happen?

there are many time that i can look back on a shake my head at the stupid things I have said or done, but I gotta say, this one takes the cake. I was sitting in a car with my friends dad and her little sister and her sisters friend. We were chatting and joking around and having a good time, Then I ended up saying "I think i met your mom a few weeks ago" My friends dad quickly said "No I think that was Kathy" being my oblivious self didn't think much of it and just got out of the car and said "Thanks for the ride, have a great game girls".  I got to school today and was spending my classes stressing about an assignment due tomorrow the involves a lot of work to be done.Walking down the hallway heading to my third period class I saw my friend who's dad drove me home. She pulled me over and told me that the poor girl that was in the car with her little sister whom I told that I had met her mother, had actually lost her mother just last year. I feel horrible and don't know what to say. Even though I know it wasn't in malicious intent and I would have had no way of knowing unless someone told me I still can't imagine what it would be like having someone remind of of the fact that you lost your mother. I feel awful for being the reason she may have gone home and cried or even just make her sad. I wish I could go say I'm sorry and let her know that I didn't know and feel awful but I don't even know her which makes it all the more awful. I guess I didn't think anything of it because she didn't seem to mind that I had said anything. So there's just one more point for me on the board of idiot things I've done. Congratulations to me.