Thank you for your query.
As you are already 20 weeks, its high time you had your second ultrasound
, which will reveal the cause of the overdistended uterus
Without doing an ultrasound, it is difficult to say why you are so big.
One reason could be abnormally large baby.
Twins could be a possibility, which was missed in the 7th week.
, that is , increased amniotic fluid
could be a reason.
Rarely, fibroids or ovarian cysts might give such a picture.
Consult your OB GYN and go for examination and ultrasound.