It's a normal fluid that is coming..however it is slightly mixed with blood..could be because of trauma due to deep fingering....cut your nails short to avoid such trauma.

Thank you for your question. A brown discharge is an indication of bleeding which is getting oxidised as it comes out of the vagina. I'm not sure why this is happening only when you masturbate unless you are in the middle of your cycle in which case it could be bleeding associated with ovulation. If you are sexually active it might help to get examined to rule out an infection on the cervix which can cause bleeding in between periods. Best wishes