What Causes Bleeding After One Week Of Periods?

Im 14 years old, and usually have a pretty predictable menstural cycle. I just finished my period last week, so I was really surprised when i found out today that Im starting it again... however, so far the flow is lighter the blood is mostly brown/dark brown. its gradually becoming more of a reddish color, but im worried about why this is happening only a week after i finished my period. i know this has nothing to do with being pregnant, as i in no way put myself in that situation. is it perhaps old blood & something i shouldnt worry too much about ? this did happen about 4 months ago, but i just really want to be aware of what it really is. pleas reply back asap! thank you!