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It might be some type of contact allergy from the grass or something in a grassy area.
I would continue doing what you are doing, and maybe add some dog booties to the mix. These can be purchased at most pet stores or online. If it is a contact allergy then the booties should help.
I might also think about the future as once allergies kick in they are likely to last for the year or for the season, so making a run area for the dog with ground cover such as wood chips, pea gravel or a cemented pen may be something you can invest in in the future.
Some antihistamines work better on some dogs than other dogs. Which are you using?