Some dogs may get more aggressive if you take away that something that a dog is possesive of some might not. It depends on the individual dog & owner & how you train.

She may be associating the rawhide with trouble, instead of the growling, especially if she not as willing to take the treat.

Is there any chance your foot may be causing the problem. Maybe the kennel people shoved her food bowl or treats in her kennel with her feet or something????
From what I read it seems as if she is more protective when feet are near.

Since the kennel owner is also your obedience instructor I would get right on the phone with her & explain the situation.

Maybe try taking the rawhide away totally for a week or 2, to give her time to maybe forget about the incident. Then hopefully she will take the rawhide with enthusiasm and you can start from scratch again.

Also if she already has a quiet command that would be great, use that when she growls instead of NO of whatever you say to punish her. That way it will help her with the confusion, that rawhide is good, growling is the bad part.