I'm trying to understand field coupling in QFT in more detail, using a free electron as the simplest example. I understand that an excitation in the electron matter field couples to the EM field to create a local EM excitation (in approximately Coulomb distribution) which can be interpreted as an 'attached' EM field. The 'attached' EM field self-interacts with the electron matter field creating a subtle change in the total mass. Also, an external EM field couples to the electron matter field to induce a force/acceleration. My question is what causes the separate behavior between the self-interaction and acceleration cases of coupling between the EM and electron matter fields, given that it's really just one EM field in both cases? Will an external EM field induce a (small) change in the total electron mass through feedback between the two fields?