molly swimming upwards

my balloon molly keeps like leening back and just laying there facing up for a couple of secs then she swins for a min before doing it again

i have done a water change yesterday evening

she is pregnant (i think)

any ideas

 

?swim bladder disease

 

i dont think so cos she only does it a few time and then shes normal

she is eating very happily
her poo is normal forming and colour

 

also she moves all around the tank quite happily

could it just be her showing off to her partner?

 

Is she gasping at the surface?