Three yr old female dog won't stop pooping in her crate?

I am hoping someone else has experienced this problem and may have some advice. Our vet has no advice on this as nothing is medically wrong with our dog. Our dog is three and female spayed mixed breed dog that is completely potty trained. She used to be on Iams lamb and rice dog food but that just got too expensive so I switched her very gradually, like over a month or two, to Costco's Kirkland Brand lamb, vegetable, and rice food (which according to dogfoodanalysis is better than Iams). It seems ever since we started the slow transition over to the other dog food she refuses to poop outside almost ever morning and when we come home later that day from work, she has pooped in her crate and has eaten all or most of it (possibly to hide it?). I just am at my wits end. Before we go to work she has like 15 to 20 minutes outside to go and she will not...our other female dogs by the way have no problems going and they never have accidents in their crates. We've tried giving treats when she is good, then discinple when she poops in the crate, given her a kong with a treat inside to occupy her time....nothing works! Does anyone have any suggestions. Is it possible for a dog to just be stubborn and poop in her crate on purpose because she wants her old dog food back??? We've also tried not spending extra time outside with her in the morning and try and make her go within the same time as the other two dogs, thats no luck either...doesn't matter if she's outside for 5 or 20 minutes, she will not go poop. And I know she can hold it, she never does this at night, only when we are at work...never on weekends either.