Agreed. If you didn't make the decision to fantasize or ejaculate, then it isn't a relapse.
Your brain and body will do a lot of readjusting because it is so used to getting porn/masturbation on a hyper regular basis (I say hyper regular because we never evolved to be so stimulated), so expect weird things. This is basically a flashback. They're involuntary, though a person can let the thoughts stay once they barge in uninvited and that can lead to a legitimate relapse. That isn't what happened to you.
Day 50 is pretty darn good. Good luck!