This paper presents a fluid structure interaction (FSI) model for predicting the side load level in an overexpanded nozzle within a forced oscillation regime. The objective is to improve the only stability model used for predicting aeroelasticity in rocket nozzles. The model in question is currently invalid as regards rigid body motions. The FSI calculations exposed in this paper point out a critical frequency of oscillation of the nozzle at which the shock motion and the side load level observed have maximum values. This corresponds to the phenomenon of a transversal wave in the flow between opposite walls, as confirmed by characteristics theory. Finally, it is shown that the parietal pressure variation in the supersonic part of the flow (upstream of the shock) cannot contribute to the generation of side load, and may be precisely estimated using a 1D-piston analogy.