The detection back-action phenomenon has received little attention in physical, chemical, and biological sensors based on nanomechanical systems. We show that this effect is very significant in ultrathin bimetallic cantilevers, in which the laser beam that probes the picometer scale vibration largely modifies the resonant frequencies of the system. The light back-action effect is nonlinear, and some resonant frequencies can even be reduced to a half with laser power intensities of 2 mW. We demonstrate that this effect arises from the stress and strain generated by the laser heating. The experiments are explained by two-dimensional nonlinear elasticity theory and supported by finite element simulations. The found phenomenology is intimately connected to the old unsolved problem about the effect of surface stress on the resonance frequency of singly clamped beams. The results indicate that to achieve the ultimate detection limits with nanomechanical resonators one must consider the uncertainty due to the detection back-action.