Thanks for your query.
In your case two possibilities may be there.
1)A condition known as blighted ovum
in which pregnancy test is positive and USG shows a sac with no foetus inside the sac and body doesn't know it is not carrying baby.May be you are in this condition but symptoms of miscarriage
are not there.
2)pseudocyesis(spurious pregnnancy,false pregnancy
) in this condition sign and symptoms are suggestive of pregnancy like absense of menses,increase weight gain,increase abdominal girth but causes may be psychogenic or endocrinal.
You should consult a doctor for your problem.
Everything will be fine.