  We present optical observations of the redback millisecond pulsar PSR
J1048+2339, which is a 4.66 ms radio pulsar in a compact binary with an orbital
period of six hours. We obtained high-quality light curves of PSR J1048+2339
with the Lulin 1 m Telescope. The system shows two distinct six-hour orbital
modulations, in which an ellipsoidal modulation changes into a sinusoidal-like
profile in less than 14 days. In addition to the change, the brightness of the
companion increased by one magnitude, suggesting that the latter type of
modulation is caused by the pulsar wind heating of the companion and that the
heating became dominant in the system. While the changes are not unexpected,
such a timescale is the shortest among similar systems. We performed modeling
analysis to extract the properties of the system. We obtained a derived pulsar
mass of 2.1 M$_{\odot }$ and a companion star mass of 0.4 M$_{\odot }$ for the
system. The irradiation power increased by a factor of 6 during which the
pulsar wind heating dominates. We also report on the two archival Chandra X-ray
observations and discuss several possibilities that might cause the varying
heating on the companion.
