I Had Unprotected Sex 6 Months Ago (15/10/2015) And Everything

I had unprotected sex 6 months ago (15/10/2015) and everything was going fine till 01/02/2016 when i didnt get my period and when i went to gynocoligist they made urine pregnancy test that came negative me vaginal ultrasound and they found no baby but a cyst ... I took medications and then got my period then it stopped again ... could be a chance that the baby couldn't be detected or seen on ultrasound and i can be pregnant ? I have no big belly till now !?