Hi,
Brown discharge is usually the result of old blood. An irregular period or delayed (late) period can often shed older tissue that will begin with a darker burgundy or brown appearance because of the older cells being expelled during the start of your period. Bleeding during or after masturbation shouldn't be considered normal; you either need to use a water based lubricant, or be more gentle during masturbation. If you've tried both options before, but continue to bleed from masturbation (or from sex), you should see your doctor for a pelvic exam to determine what the cause is.
Take Care