What Could Be The Reason For Delayed Period When Not Sexually Active?

am 26 yrs old unmarried girl. i always hafd regular periods with in 26 to 32 days. but this time i am not having my periods and it is now late by about 12 days. this is the first time in my life. what will be the reason. when should i see a doctor. what test will a doctor do. i masterbate sometimes . will the doctor do pelvic test. wil she understand that i masterbate. . wikl she tell that to my parents