Gray spot on teeth?

My baby girl has had her two bottom teeth since she was 4.5 months old. Today I noticed that where the teeth meet, there is a gray spot. It almost looks like something is stuck between them, but there isn’t anything there. I did some Google research and the only thing I could find was that it could be from iron. We brush her teeth every day and it didn’t come off after her morning brushing or her evening brushing. I’m going to call the doctor tomorrow if it’s still there, but has anyone else experienced this?