We have performed muon spin rotation and relaxation $($μ${}\mathrm{SR})$ measurements on single crystals of the chiral helimagnet ${\mathrm{Cr}}_{1/3}{\mathrm{NbS}}_{2}$ at zero to low magnetic field. The transition from the paramagnetic to helical magnetically ordered phase at zero field is marked by the onset of a coherent oscillation of the zero-field muon spin polarization below a critical temperature ${T}_{c}$. An enhancement of the muon spin precession frequency is observed below $T$∼${}50 \mathrm{K}$, where anomalous behavior has been observed in bulk transport measurements. The enhanced precession frequency indicates a low-temperature modification of the helical magnetic structure. A Landau free-energy analysis suggests that the low-temperature change in the magnetic structure is caused by a structural change. We also suggest a longer periodicity of helicity below $T$∼${}50\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, which can be verified by neutron-scattering experiments.