Hello and welcome. The original expert you asked for is not in the dog category any longer. My name is ***** ***** I have been working with dogs for 25 years. I am a Therapy Dog & AKC Evaluator/trainer/ specializing in behavior. It will be my pleasure to help you today.
From a behavior standpoint , something in the grass either hurt her or frightened her, known or unknown to you. I do not believe it has to do with the UTI.
You may need to counter condition her to the place she fears which needs to be done is small steps and not just for potty reasons. She is learning nothing from going for a mile walk other than to hold it longer until she gets home. Whatever happened while she was on grass is what is burned into her mind. It could have been a bee sting, or a loud noise in the neighborhood that set it off. Try to think back to unusually things happening in her life at the time it started. A UTI is from bacteria so it is hare to tell if the uti came from her licking from failure to properly pee outside or if it was just coincidence.
Counter conditioning should be done throughout the day in brief sessions and gradually you increase the time she is on the grass.As I said,m do not make it for potty outings only, spend some time getting her engaged in what she likes to do for brief moments, and take her back inside before she gives thought to being on the grass.
When she is inside the home and you cannot watch her then she should be crated, tethered to you, tethered to a spot near her bed and food dish, or kept in an area that can be easily cleaned so it does not aggravate you. If she sees you aggravated at cleaning up her mess she will never want to go in front of you again and this will lead to more problems.
I am going to direct you to a link on how to counter condition in scary places .