Anytime, you feel any cramping that seems to feel abnormal to you, call your care provider. Your own intuition can be powerful and listening to that feeling may save your baby's life. Oftentimes preterm labor will feel like menstrual cramping or a low dull backache. Even if the pain doesn't come and go like contractions normally would, it should still be considered a warning sign. In a normal pregnancy the uterus contracts regularly as exercise in preparation for childbirth at which time the cervix opens and the baby is born.
Preterm labor progresses the same.
The difference between contractions that are normal and those that lead to delivery are that the cervix goes through changes. And there is no sure way to know this without having a vaginal exam done. Such exams should be limited as they have the potential themselves to cause the uterus to contract even more. Other signs of preterm labor include different or increased vaginal discharge or any bleeding which happens when the cervix dilates. Also, low pelvic pain and rectal pressure can be a sign that the baby is descending in the pelvis and eager to be born.