Toddler teeth - cavity??

We brush our daughters teeth 2x a day but they are starting to turn brown around the gums! I’m horrified. She has her first dentist appt next month and I was only able to move up the appointment by 6 days 😩. She’s 20 months and still breastfeeds so I’m not sure if that has anything to do with it? Has anyone else’s kids had teeth like this?