missed periods

Hi i wanted to know if people can help,

In january 2013 i gave birth to a 2  month premature baby boy weiging 3lbs 2oz, i bled for 6 weeks after then my periods started everymonth from march then in july i noticed my period stopped again, i took a pregnancy test it came back negative so i put this down to stress with a premi newborn, then a month later i still hadnt received my period so took another test and stil negative, were now in october amd after seing my doctor who has refered me for a scan in a weeks time im scared at what it could be can anyone help or give advice,  i still havnt received a period since july and gettin very worried.