Bleeding after external masturbation

Hi, I'm 21 years old and while I don't currently have a sexual partner, I am not a virgin. Up until about a week ago I had never had any problems with menstruation, other than the normal cramps and everything. I'm on birth control so it's usually regular, but lately almost immediately following masturbation I have some vaginal bleeding. I wouldn't call it a small amount but it isn't quite as much as a normal flow and I have to use feminine products the next day because I'll have the brown discharge like I get at the end of a period all day. What confuses me is that I'm not doing anything internal, I'm only doing external masturbation and I'm just very confused. Should I be concerned?