What Causes Irregular Periods With Spotting And Clots?

My Son is 6 months old so my periods have been irregular. January i got my period. 2 weeks later i spotted dark brown for 2/3 days and then 2 weeks later i got my "period" again. its not blood though its like a Dark red mucas it hasnt been enough to fill a tampon or a pad it just comes out in clumps when i pee. *i took a pregnancy test it says negitive* so what the heck is going on?