Your creature does not sound like it follows the laws of conservation of momentum and energy. Remember that, just because you "ate" something doesn't mean its momentum goes away. You still have to expend energy and propellant to offset that momentum.
Also, the closing velocities for most space collisions (which is what eating has to be) are typically very very fast. Like "makes fighter planes look like their standing still" fast. A F-16 tops out at 1350mph. Micrometeorites are known to cause issues for spacecraft by traveling at 22,500mph or more. At these speeds "eating" a rock is more like eating a bullet from a revolver. You would be obliged to match its speed first, then eat it, then move. This means a tremendous amount of energy expended accelerating and decelerating your own body.
The only way I can think of to generate that kind of energy is if this creature has somehow figured out how to do cold fusion of atoms which are too heavy to be fused in stars. Even then, it would be the single most energy inefficient "animal" ever devised. Not even we humans are that wasteful.
In all, I'd recommend walking away from the reality-check on such an animal, and just explain their behavior with magic.