I believe that you can only get chicken pox once. After you have had the infection the virus is present in every cell in your body for life, but is non-contagious, or re-infective as you're immune system keeps on top of it.
Over time the cells in your body are replaced, except for the nerve cells, (and brain cells). If you get a bit run down - well in fact alot run down, you're immune system is unable to keep the virus in check, and it flares up again in the cells where it is present - the nerve cells. It is known as shingles.
As the nerve cells are sensitive to pain, they react to the infection, and essentially hurt like hell.
Elderly people are more prone to it, due to general physical run down associated with age. My mother-in-law manages a rest home, and she says she has seen a resident, who was a wounded, decorated WWII soldier, stiff-upper-lip type of person, cry like a child because the pain is so intense. He was prescribed morphine.
Alternatively, you might not actually have had chicken pox as a child and this is your first infection.
Good luck