Deadball PF vs Roughing the Passer - help

[NWMichiganRef]

Fed Rules:

Had this situation last friday night and I need some help on this call as I am going back and forth. 3 and 19, QB drops back to throw and in the middle of his throwing motion he is contacted by a defensive player causing his pass to fall incomplete 10 or so yards away. I hear the whistles from the LJ and BJ so I immediately start blowing mine as the defensive player is still locked up the QB driving him backwards. Despite the whistle the defenseive players drives the QB another 5 yards and than throws the QB to the ground. I flag the play immediatley, however, instead of giving myself a second to think about what occured I wheel around and give the signal for deadball personal foul and stated that it is 15 yards from the previous spot resulting in 4th and 4. My mind was telling me that this action was a good 2-3 seconds after the whistle thus it is a deadball foul. However after thinking about it should I have called roughing the passer which carries the automatic first down? After thinking about this over the weekend I have come to the conclusion that I messed this one up and should have called roughing the passer.

On a side note, if I would have called roughing than I would not have had to explain to the coaches that personal fouls for late hits don't give the offensive team an automatic first down. But that is another long story.