What Causes Brown Colored Vaginal Discharge?

Im 22. in good health no problems but recently started getting a dark brown discharge. i had my period 2 weeks ago and so i dont know if its just spotting in between periods or what. my periods used to be regular but have been all out of whack since i had my son a year ago. is it something i should be concerned about? today is the 2nd day of me having this. there is no pain or anything like that. i know for a fact it isn't an std because i'm married and my husband has only ever been with me.