I'm not sure you can be "sure" it's the chicken pox -- I think that's a good question for your doctor. I wonder if a blood test could show that. I do know lots of kids who have gotten chicken pox -- and they all have been vaccinated. It went through our school of vaccinated children last fall. My doctor was always anti-vaccine because of just this. Plus he thinks 20 years from now we're going to see a lot of adults with chicken pox when the vaccine wears off. I had my daughter vaccinated when I was forced to for school. And I also think she had a mild case this year but it is hard to know. (She's 10)
Just an add on -- one poster said it isn't as bad as they say when you get them as an adult. I disagree! I had them in college, age 21, and I ended up in the hospital where I was told that I had the worst case they had ever seen. I had them in my throat, which made it difficult to breathe. And I was down and out for two entire weeks. It might not be that way for every adult, but it was horrible for me, and my older daughter (20) who got them at age 2, doesn't even remember them.