contraceptive depo injection....i had two major operations

Hi i was wondering if someone could help i had the depo provarda injection once on the 13th jun 11 and was due to get another one on the 5th of september which i didn't.... my problem is that while i was on the depo injection i had to major operations and 2 courses of antibiotics.... could this interfer with the injection as i have had a lite spotting and i passes out for no reason. i have read that the depo injection stays in your system for 14 weeks not 12 and this monday i would have been clear of the shot for 3 weeks.... could i be pregnant???