What Causes Brown Discharge After Masturbation?

So my husband and I had sex and then a couple of days later I started my period. I was on it from Dec 27th until last tuesday. a couple of nights ago I masterbated just clitoris nothing inserted and woke up with a brown mucus discharge when I wipe. It went away later that day and came back a couple of nights later when I masterbated again. I have no pain or anything and have take a HPT that came out negitive. What do you think this is?