Does she have balance issues at other times, with other activities? If she just ate, don't handle (don't try) this right away, let her digest for at least a day or 2. But see if she has trouble righting herself when you gently pick her up & place her on her back? Does she have trouble hanging onto you (your hands/arm)?
Depending on her genetics, this balance issue might be "normal" for her, but it also might not be, sofor noticing & for asking.
How long have you had this snake? What morph is she, if any? Did you get all your BPs at the same time & from the same source, or from different ones? Did you quarantine them, if not?
This might be nothing more than a strength issue- snakes neck muscles are designed to constrict prey, not to support themselves vertically, so yes, the weight of the prey might be causing this.
There's no "law" that says all snakes (even the same age) will be created equal, just like some children are stronger & grow faster than others. Hopefully this is just her "normal".