Well, apart from the obvious
big fat Dr. Momma titties suckie suckie milk......
*Ahem*, I would presume there was some form of a congenital defect that she received at birth, or perhaps some other type of abnormal mutant power that is internally focused... so, perhaps the ability to transform her body at will (
preferably her titties), or mayhaps she might have some other hidden power beneath her external facade....
Or we may simply just be overthinking it, and Pepper is just a simple doctor tagging along the mutants for her own personal research gains if anything.....
But I want her milk to go with my coffee 
, ahem but yeah interesting to speculate about nonetheless....