What Causes Thick White Vaginal Discharge?

My name is amanda I had my last menstrual cycle may 8 and i didnt start my period this month in june i was suppose to start yesterday june 8, i havent been experiencing any symptoms of pregnancy but iam scared that iam also yesterday i had a thick white discharge come out of my vagina and i dont know what that means, Iam 21 years old 60 inches tall and weigh about 122 pounds, I had a miscarriage febuary 2010 PLEASE HELP ME im worring my brains out, i dont want to be pregnant,