What Causes Dark Brown Clumpy Discharge?

I'm 15 and I've had my period for almost two years. Sometimes I get quite dark brown clumpy discharge. Just like normal discharge just coloured brown and slightly heavier. I would say it's too heavy to be spotting but not heavy or long enough to be my period (it last around 3 days). I've read that spotting in the middle of your cycle can be something to do with ovulation but this seems to heavy to be normal. I'm not sexually active and this has happended to me a couple times before. Is this normal? Thanks.