  We give a detailed description about the reflected spectrum of a few-cycle
pulse propagating through a resonant dense medium. An unexpected low-frequency
spike appeared in the red edge of the spectrum. To figure out the origin of
this redshift spike, we analysis the mechanisms responsible for the redshift of
the reflected field. So far, the redshift has not been well studied for
few-cycle pulses except a brief explanation made by the previous study
[Kaloshan et al., Phys. Rev. Lett. 83 544 (1999).], which attributed the origin
of the redshift to the so-called intrapulse four-wave mixing. However, we
demonstrate numerically that the redshift consists of two separated spikes is
actually produced by the Doppler effect of backpropagation waves, which is an
analogue effect of dynamic nonlinear optical skin effect. Our study elucidates
the underlying physics of the dynamic nonlinear optical effects responsible for
the redshift spikes. Moreover, the dependency of the their frequency on the
laser and medium parameters, such as medium density and input pulse area are
also discussed.
