Thanks for your query.
Since your home pregnancy
test and blood pregnancy test, both are negative, I think you are being paranoid about pregnancy.
About the odd butterfly feeling , that could be due to plain anxiety
or a minor muscle flutter that you are experiencing.
The belly size could be due to simple weight gain.
I think you are very anxious about the pregnancy, inspite of having the coil fitted, so please see your gynecologist who would examine you, and perform an ultrasound
and reassure you once and for all.
however, your chances of being pregnant with negative blood test
is practically zero.
Take care and relax.