Mary's totally right--dogs can not only be allergic to pollen or grass, but also to just about anything that humans can be allergic to. When we were trying to determine the cause of Kobie's serious coughing a few months ago, one of the things the vet mentioned was doing an allergen blood test. It's very expensive and doesn't always pinpoint the allergy, but I was amazed at the lab paperwork for it--the blood is tested for reaction to everything from dust and carpet fibers all the way to specific molds, grasses, flowers, trees, you name it. (Kobie ended up being diagnosed with non-specific allergic bronchitis, so she takes a broncho-dilator pill to help her breathe.)

Benedryl usually works well for grass/pollen allergies. The good news is that your dog's problems might be seasonal and/or location-specific, so if you can keep track of the weather, time of year, and environmental stuff that seems to set her off, you might be able to predict when she'll flare up and try to prevent it with meds. What a happy pooch she'll be!