Is the blood coming from inside your vagina or outside? Are you using a method of masturbation that would cause bleeding? Such as, something sharp or that pulls hairs? (I'm trying to picture what that would be but unless you were sticking a bobby pin up your urethra it's hard to imagine.) And does the bleeding relate to the time of your period, like there was possibly some last period blood? Also, are you putting something up inside you that might bonk your cervix? (They can be tender and hang lower at certain times of the cycle.)
Why can't you go to your mom? Do you have a place such as Futures, or Planned Parenthood? (Those are resources in my town, I don't know about where you're from). If something is wrong, you need to go to your mom because what if it was something major and you never get it treated? I know it might be scary, but take this from someone who had to tell their mom about how I had sex at 13. It's SCARY, but honestly, it'll show your maturity because when you needed help, you asked for it.