The question about ABAQUS element: CPS4R

The element is added displacement load in one direction, at the steptime=0, the nodal displacement is zero, and so as the integrated points' strain and stress. However, the internal force NFORC is nonzero. When we calculate internal force by ourselves, it is obtained by the integral for BT*stress. But the stress is zero, how Abaqus calculates the nonzero initial internal force?

The details can be found  in the attachment.