Should I reach out?

I saw this girl I used to work with at my former software company at the local grocery store tonight. I haven't seen her in about 6 months roughly. We always were polite to each other and I'd catch her staring from time to time and had nice conversations, nothing crazy but just typical colleague small talk. I always felt a different vibe around her and one that I was never able to ignore..even to this day. So we exchanged smiles as we passed each other and then it clicked to me who it was after I left that aisle. I tried to find her to say hello but she was gone. Is it too awkward for me to message her on Facebook (I never added her) or should I just leave this be?