Hi B.,
My son had the same thing on his front tooth around the age his teeth grew in. You can't get into the dentist until they're 1 yr of age and when I took him the dentist said it wasn't cavities it was lack of enamel and it formed like that when he was in my womb.
She said if it was caused from bottle feeding the spot would be higher up on the teeth, not on the bottom part. One thing to check for is make sure the yellow spot does not become soft. If it's still hard when you touch it (like you can't put a toothpick through it, is how the dentist put it) then it doesn't need to be fixed right away.
Just make sure to keep it clean like you're doing and you can get a toothpaste made for infants in the toothpaste isle. It's perfectly safe if she swallows some but just use a very small amount.
When she turns 1, have her seen by a dentist but don't worry yourself about it! She'll be just fine and my son is 21 mths old now and still has his yellow spot on his tooth. The dentist says as long as it's not soft then there's no point in putting him to sleep so young to fix it.
Take Care!