22 with chicken pox... but i had them when i was 2?

Im so confused! i went to my dr yesterday because i had a very sore throat and i was scared it was going to turn into meningitis again (im paranoid because i got it 5 weeks ago and only recovered 2 weeks ago) well he goes my throat is very swallen and red but he thinks it viral again and i said i have also got a rash all over my arms chest and back that i noticed about an hr ago and its so itchy he goes you have the chicken pox. I was stunned! i told him i had them when i was 2 and that i thought adult chicken pox came out as shingles he said no and that when i was little i must have only had a small dose of the virus. Just wondering how long do they usually last? and how long am i contageous for? i have uni exams in a week!!