We characterize Y/T-system type difference equations arising from cluster algebras by triples of matrices, which we call T-data, that have a certain symplectic property. We show that all mutation loops are essentially obtained from T-data, which generalizes the general solution for period 1 quivers given by Fordy and Marsh. We also show that any T-datum associated with a periodic Y/T-system has the simultaneous positivity. As an application, we propose a version of Nahm's conjecture from a viewpoint of cluster algebras. We conjecture that given a periodic T/Y-system of a certain type, we have a family of hypergeometric q-series that are also modular functions.