What Causes Missed Periods After Having 2 Menses Last Month?

Hi, I had tubectomy in July 2011 after my 2nd c-section. After the initial post partum bleeding, I had my periods twice. This month I had slight spotting on the 24th day but did not get my periods. Also had mild cramps and experienced slight backache and headache. My periods have been normal and was occurring on 27 day after delivery. Pregnancy test negative on 27 day. no cramps today on 30th day.
Please tell me What is happening.