It's possible that your period is coming on. Is it earlier than you expect it? Periods can be a bit irregular at 13, and can actually come early or late just because.
It doesn't sound like you have done anything (regarding masturbation) that would cause you to bleed. If you had said you were using something that had sharp parts or bristles and had put it up your vagina, then that might suggest you irritated your cervix and caused it to bleed. But it doesn't sound like you've done anything like that at all. So, the logical guess is that your period is coming.
Write back if your period doesn't come but you merely keep lightly spotting, and we'll try to figure out if it's something else.