We present multi-epoch optical spectra of the gamma-ray bright blazar 1156+295 (4C29.45, z = 0.729) obtained with the DeVeny spectrograph on the Discovery Channel Telescope of Lowell Observatory. During an unprecedented multi-wavelength outburst in late 2017 when the optical flux increased by 4 magnitudes, the peak gamma-ray flux exceeded 2×10-6 phot/(cm2 s), and the quasar was detected for the first time at energies greater than 100 GeV, the flux of the MgII 2798 Angstrom emission line varied, as did the iron line complex at shorter wavelengths. The line fluxes were proportional to the variable optical continuum flux, which is presumably synchrotron radiation from the relativistic jet. Both the line and continuum fluxes rose and fell together, suggesting a causal connection with a time delay less than about 1 week. This implies that the line-emitting clouds lie near the jet, which points almost directly toward the line of sight. During high-flux states, the MgII line displayed a prominent red wing, which could be produced by inward-falling clouds located about 1 pc from the black hole. This research is supported in part by National Science Foundation grant AST-1615796 and by NASA through Fermi guest investigator program grant 80NSSC19K1504.