Thrush Mouth

I'm so nervous and scared my cookie is 4 months old and this morning I woke up this morning and I gave her a bottle and I looked at her bottom lip I thought it was milk but it was a white patch then there was one on the tip of her tongue. Is it thrush? Where did this come from I'm thinking about going to emergency room but my husband told me to just call the doctor first thing in the morning to ask if she can be seen. What should I do?