Structure formation during flow, a common phenomenon driving polymer processing, can have a major effect on the rheological behavior of a polymer melt. This can cause large increases of the pressure and thus large changes in thermodynamical properties which, in turn, will strongly influence the structure formation. Consequently, a complex, mutual, self-influencing process, occurring at rather short times, arises. This process is investigated with combined in situ small-angle X-ray scattering and wide-angle X-ray diffraction at high acquisition frequency (30 Hz) in a piston driven slit flow device using linear low density polyethylene as a model material. A decrease of crystallinity is observed immediately after flow and related to an unusual melting of part of the oriented crystals. The experimental observation is explained in terms of pressure dependency of the undercooling. The undercooling first increases during flow because of the pressure rise and then drops when pressure relaxes to equilibrium values; as a consequence, the critical stable lamellar thickness is not constant in time, although the experiments are conducted in isothermal conditions. A mechanism is proposed and validated using a structural model to fit SAXS data: the increase of undercooling during flow promotes nucleation of gradually thinner lamellae on the pre-existing kebab nucleated from shish cores that relax back to the melt state after depressurization. Our results show that in modeling real-life processes involving combination of flow and high pressures, like injection molding, the effect of pressure on the shear layer formation cannot be neglected.