We present single-shot measurements of the longitudinal photon source position of the SPring-8 Angstrom Compact Free Electron Laser x-ray free electron laser by means of x-ray grating interferometry. The measurements were performed in order to study the behavior of the source under normal operation conditions and as a dependence on the active undulator length. The retrieved experimental results show that x-ray grating interferometry is a powerful in situ monitoring tool for investigating and tuning an x-ray free electron laser.