If you are young and only just starting getting a period, this is nothing to worry about.
It's normal for your first period to be over quickly, so this might explain why you have already started another bleed. It might even take several months for your body to get into a predictable pattern.
Every girl is different and they can last for anything from three to seven days but typically you will have a period every twenty eight days.
If you are still concerned, go see your local nurse to put your mind at ease. It's normal to worry too!