help! is this period blood?

Ok so I've been having discharge for a while. It's always been healthy though. Just yesterday, I noticed a little bit of something brown in my underwear. Not knowing what it is, i did nothing. (Keep in mind that if this is period blood, it's my first period.) Today, there was more brown stuff. It is toward the from of my underwear and smells like discharge , but looks the color of poop. Is it period blood? At first it's a little lighter brown, but then becomes dark. Please help!!!!