What Causes Missed Periods, Spotting With HPT Negative?

Hi, may I answer your health queries right now ? Please type your query here...am not getting hpt positive after one week 2 dys of missd period,i had bleeding little on 16th fab.till 18th,with spotting,
my last period is 23 jan 2011 ,till now i have not got my periode am breastfeeding also my son 4 years old, i have regular periods with 29 days cycle.