plaque, tartar, something else??

So I noticed this blackish brownish stuff between my teeth recently and I can't seem to figure out what it is I really don't want to go to the dentist before I know what it is so I can try and get rid of it myself without having them scrape my damn mouth and I brush twice a day and floss regularly so I don't understand why this is happening.. I started using a new toothpaste..could that be it? I've never had this before so I'm not sure what it is if anyone knows can you please tell me? Thank you so much!!!!