Just double checked this as I thought the fluid from the little blisters also spreads chicken pox... did a google search and this is from the Better Health Channel website (Australian).

Chickenpox is spread by air-borne droplets from the upper respiratory tract (coughing, sneezing) or from the fluid in the skin lesions. An infected person is contagious for 1–2 days (possibly five days) before the onset of rash and remains infectious until the blisters form scabs (usually around day five of the illness).


High risk groups
People at increased risk include:
Newborns – there is an increased risk of severe illness in the first 28 days of life.
Pregnant women – who have not had chicken pox or been vaccinated for chickenpox. If a pregnant woman acquires chickenpox in the first 20 weeks of pregnancy, there is risk of congenital varicella syndrome and this can cause abnormalities. After 20 weeks the risk is lower. However, having chicken pox around the time of delivery (five days prior to or within two days after delivery) can result in a high mortality rate of the newborn.
People who have depressed immunity, such as those with leukaemia or Hodgkin’s disease, or those taking immunosuppressive drugs are at risk of longer and more serious illness.

I know you are worried, but hun, there's truly nothing you could have done about it when you didn't even know she had chicken pox. I know that's easy to say and I'd feel the same as you are, but it isn't your fault.