Worried......

I found out I was pregnant back in January when I went to my 2nd doctors appointment when they did the ultrasound they didn't see anything in the gestational sac. So they sent me over to the hospital and they did another ultrasound they didn't see anything either. Well long story short I had a D&C in March. Well now it's September and I just found out I'm pregnant again. What are the chances of the sac being empty again? Is something wrong with me? Am I just not fertile?