in my experience if a dog is gonna guard something 9/10 times it will be rawhide. even those dogs who don't resource guard otherwise. for this reason my dogs don't get rawhide.

if you want to continue to give heidi rawhide treats then i suggest that you keep hold of one end. so the rawhide belongs to you and only by your grace and benevolence are you allowing heidi to chew on it. it is never hers. you are the controller of treats esp rawhide. whilst you are holding the rawhide and she is chewing it i would ask her to out or give or whatever you use and then immediately allow her to chew again. this way she learns that it is not so bad to relinquish the treat as you will likely give it right back. practice a lot.

if she begins to guard other things like places, people, toys etc then i would suggest the book "Mine!" by jean donaldson.