What Causes The Presence Of Gestational Sac With Absence Of Yolk Sac?

My first pregnancy was a miscarriage due to me lifting a heavy object i think. i am now on my second pregnancy and i am around 7-8 weeks if you go by my last period. My first sonogram showed a gestational sac with no yolk sac in it. my second sonogram showed the first gestational sac measuring 1.47 cm still no yolks sac and then a second gestational sac appeared much smaller no yolk sac in it either. what does this all mean? did i loose the baby early on and what is this second sac?