I have a little painless cyst like blister inside the skin of my penis shaft just under the region where foreskin meets shaft. Is it cancer?

See your doctor. You need to see your doctor to have it formally diagnosed and treated.
Infectious? I would think of infectious causes before i would think cancer. I refrain from sexual relations and see your friendly primary care doctor.
Unlikely but. At your age, penile cancer is highly unlikely but would suggest you to see your primary care with possible referral to urology.