It could be an allergen (eg pollen, plant moulds) that’s just in that local area, which would explain why it’s come on suddenly.
Insect bites can cause itchiness but it’s not the only possibility.
Some years ago my hubby and I visited Edinburgh in the autumn and he was suddenly afflicted with a major allergic reaction (eyes streaming, sneezing, coughing, headaches). As soon as we left the Edinburgh area he was fine. It was something in the air, just in that area.
When affected in the same way by airborne allergens dogs don’t get respiratory symptoms, they get itchy and red skin and eyes.