What Causes Blood Traces In Vaginal Discharge?

Dear doctor.i am a lactating mother since 2 yrs.My periods are irregular since 4 months. and during some months I dont get per periods .both urine and blood pregnancy test is negative, After a gap of two months I got my last period on april 5th. I usually have white discharge.now I could find some discharge containing blood strands .I have vigorous back pain too.can you pls tell me what is the reason doctor?