You could be experiencing contact dermatitis, which is a reaction to any chemical, plant, animal, you may come in contact with. You also may just be breaking out because of the sweat and dirt you are coming in contact with daily. Try exfoliating in the shower once or twice a week and see if that helps. You might be allergic to chemicals in the grass, or grass itself. You can try going to an allergist to be tested and you can be put on medication specific to your allergies.