Woke up with brown stains on my tooth? (Photos)

Hey! I check my teeth regularly as I am very concious of them. I have good oral hygiene: brushing twice a day, using mouthwash twice a day, oil pulling twice a day and using floss once a day. However, this morning I noticed that there are a couple of brown stains on my first maxillary right premolar. These stains were not here last night:( I've booked an appointment at my dentist. Does anyone know what this could be I.e. a cavity or staining. Can it be removed at the dentist, is so how? Thanks!