given the time of year, and the fact the snow has been sitting on the ground for so long, it may very well be mold or spores that are releasing as the snow melts and the ground is exposed. I ended up miserable when I went to university every spring as the snow melted. The doctors saw it every spring and were used to people lining up at the clinics in town with with they affectionately nicknamed "snow mold" allergies. It was bad for people with reduced immune systems as it manifested as bronchitis, pneumonia, and a hospital stay.