We can't say for certain what cause the bleeding, but given that it happened after you had your fingers inside your vagina, the most likely explanation is that you gave yourself a small cut or tear. That can happen in you're fingering yourself roughly or intensely, if you're not using a lot of lube, or if your fingernails is a bit too long. You may also have had some spotting between periods and happened to masturbate at the same time.
All in all, this probably isn't cause for concern. But if you notice the bleeding happening more often and it's not your period, or you start noticing other symptoms like painful urination or weird discharge, then you'd want to check in with a healthcare provider.