What Causes Delayed Periods After Pregnancy?

i had a baby 6 months ago and had a normal period when he was 3 months and have not had one since but for the last 2 weeks i have has a brown discharge with stringy brownish stuff is the best way i can discribe it i have take 3 pregnancy test all have been negitive i am 19 and this was my first baby but have never had a regular pieriod but have never been this late should i be worryed and every now and then i get a sharp twinge for a second then i goes again for a few days