Why is my period so late? 15 years old?

i don't think it has anything to do with masturbating and it probably hurt because you've never had 2 up there before and its loosening you vagina. You need to stop worrying. It seems like the fact you period hasn't started is making you stressy, which is only going to delay it more. You're 15 and so its normal for your period to have a mixed up pattern atm. especially if you didn't start that long ago, or if you're stressed or sick. Just don't worry and it will eventually come in its own time. If you continue to worry and it still doesn't come then just speak to a doctor but im sure if you stop stressing it will be here in no time.