I'm 16 years old and I'm a virgin and I've had my period for 3-4 years now and recently I started getting red/pink/brown discharge 18 days after my last period (May 6 is the day I started my last period). It has been 4 days now since it has been happening so what is happening??

I've never had sex so I'm not pregnant. I'm not sexually active either, 101% virgin. 

Is it just ovulation bleeding or like midcycle bleeding??