Should I be contracting/cranking like I'm on my period if it's not suppose to come till the 18th? I've been sexually active on the 6th n 12th

It's possible. The prostaglandins in semen may cause an increase in uterine contractions, but if it's 2d since the last time you had sex, you may want to have your doctor confirm that something else is not causing this. This may be a sign of an early menstruation, since ovulation time can vary, and be a few days early. Give yourself some time, and if the cramping continues without a period, see your OB GYN ASAP.