Last Year In The End Of June/early July, I Had

Last year in the end of June/early July, I had a large number (around 16) of gnat bites. They became vesicular and had large amounts of drainage and covered both legs. There were no other spots on my body. I was gardening at the time and the doctor thought I had come in contact with poison ivy/oak but I hadn't. I thought that it was from the bites...which I do react to somewhat. I took Prednisone for one month and it slowly resolved. In the past 24 hours, I have noticed small water blisters again...four on one leg. When they burst, they itch similarly to last year. I do not have the bites and have not gardened this year. I do have a history of shingles. What could be the cause?