Why am i 15 and my period lasts 5 days?

First off, why are you complaining? To some women 5 days is a blessing! Every woman is different in how their cycles are. Me for instance I am lucky, mine only last 2 to 4 days on average, however mine are extremely painful. Most women's cycles last anywhere from 3 to 8 days. So you are perfectly normal. Smile now..no worries! And remember, that's what parents are for, don't be afraid to talk to your mother, she's gone through her cycle as well, and probably still does. It's a good thing to open up to her and let her give you that motherly advice.