Why Am I Not Getting Periods?

on 1 july 2011 my laproscopy had been done, after that i got my periods on 28 july 2011, after that i didnt got my periods, i did hpt also twice it came negative, after that doctor asked for beta hcg test it also came negative, it was 1, after that i showed to doctor, doctor did my sonography she said you will get your periods after 5 days, she gave medicine also for that, but still i didnt got my periods, is there any chance of my pregnancy.