Whats going on with my period?

Ok so I had my second daughter Jan 28, 2010.. I pumped for 8 weeks and than slowly declined and stopped. I had my first period started April 7, 2010.. it was a heavy period that lasted for about 4 days.. Well I was due for my second period on May 5, 2010 because my cycle is 28-29 days.. I missed my period and its now the 10th and I still have got my period. I did take a pregnancy test and it was negative so I am not sure what could be going on. my husband and i used protection and avoided my fertile window...so could my period be delayed cause of not ovulation? I have never missed a period except for when I was pregnant so I am not entirely sure what else it could be but test are negative...any suggestions? oh and i took a first response early detections test (6 day early one) 3 days after my expected period.. Any info would be great. Thank you!