You're fine. Period blood color can be a little brighter than usual, espsecially if you're seeing it on your fingers rather than after it's been on a tampon or pad for a few minutes to hours. It's really not that big a deal if you did accidentally tear it as it happens all sorts of ways before a girl actually loses her viriginity these days. Being extremely active, riding horses, etc., can all result in that happening.
If you are going to be worried like this when masturbating on your period, I would recommend to avoiding vaginal masturbation and try other ways.