Hi Redrojo, and welcome to Scarleteen.
It's possible that if you're masturbating without much lube, a sharp fingernail could create a small cut or abrasion that is somehow bleeding without you noticing pain, but in general I'd expect that to hurt or sting at least a little bit so I don't know how likely it is.
Sometimes people will experience some spotting on the pill so that could be the cause of the bleeding you're noticing. Also, there are some STIs that can cause spotting so that's something to be aware of; if you aren't up to date on STI testing, then I think checking in with a doctor might be a good call. We can't know if that's the cause or not, but it's something to keep in mind.
Are you noticing this bleeding at any other times, or just during/right after masturbation?