Your prey item, as noted, is WAY too small.
If you want, just move her up one size per meal till she is on something the right size.
These snakes grow very fast, and that is not enough food to supply her with what she needs.
As for the right prey size, not sure what size she should be on, but I am sure lots of people will chime in with the right size for a boa of that age.
My baby BPs get larger prey items than what you are feeding your boa.
Bruce