  We report on the detection of a statistically significant flare-like event in
the Mg~II~$\lambda$ 2798~\AA\ emission line and the UV~Fe~II band of CTA~102
during the outburst of autumn 2017. The ratio between the maximum and minimum
of $\lambda$3000~\AA\ continuum flux for the observation period ($2010-2017$)
is 179$\pm$15. Respectively, the max/min ratios 8.1$\pm$10.5 and 34.0$\pm$45.5
confirmed the variability of the Mg~II emission line and of the Fe~II band. The
highest levels of emission lines fluxes recorded coincide with a superluminal
jet component traversing through a stationary component located at $\sim$0.1
mas from the 43 GHz core. Additionally, comparing the Mg~II line profile in the
minimum of activity against the one in the maximum, we found that the latter is
broader and blue-shifted. As a result of these findings, we can conclude that
the non-thermal continuum emission produced by material in the jet moving at
relativistic speeds is related to the broad emission line fluctuations. In
consequence, these fluctuations are also linked to the presence of broad-line
region (BLR) clouds located at $\sim$25 pc from the central engine, outside
from the inner parsec, where the canonical BLR is located. Our results suggest
that during strong activity in CTA~102, the source of non-thermal emission and
broad-line clouds outside the inner parsec introduces uncertainties in the
estimates of black hole (BH) mass. Therefore, it is important to estimate the
BH mass, using single-epoch or reverberation mapping techniques, only with
spectra where the continuum luminosity is dominated by the accretion disk.
