What Causes Missed Menstrual Cycle?

Hello, i was on the depo shot, but I had gotten really sick in Dec. 2010 & was put on antibiotics for a week. in Jan.2011 I went to the doctor to ask about a missed period & the doc took a blood sample but the result came back negative. i was due for my depo shot again in Feb.2011 but have been using condoms instead. it is now april & i still haven't gotten a period. 3 months & still nothing, but all 4 pregnancy test are negative. I have 2 kids & never had been stressed out so much about what is really going on. am i pregnant or is there something wrong?