I don't usually do it often if anything I'm still relatively new and this is probably my 5th time doing it but I noticed in my last time (4th) that I bled (but ended up getting my period later. However I'm over it and decided to go again (5th)but when I finished there was a little bit of blood. My nails are the same length as my finger so i don't really know if I scratched myself or I did something wrong.
If it's fresh red blood then it could be that you tore your hymen a little or if it's dark blood it could be from your last period.