Whats in my Babys mouth?!

The day we were leaving from the hospital when i had my daughter we noticed what looked like a blister on her lip. We asked the on site pediatrician and she said it was a blister and to leave it alone it would go away. The "bump" would grow and shrink back down, it would become bright red and look like its filled with blood. We've mentioned again and always get the same response. Shes now 6 moths old and the "bump" is at least tripled in size and stays that way. Sometimes i think it seems to bother her though its hard to know if thats the case. I would like answers so i can help my baby girl.

Here is a picture of her "bump"