Cramps vs contractions

Obviously FTM here. Experiencing some cramping tonight, like menstrual cramps very low with some back pain too. Not extremely painful but definitely noticeable. The question is - would I definitely know if I was having contractions? I imagine a contraction to be like really painful tensing and then a definite beginning and end - that might be totally wrong. Someone educate me! 37w today - baby is dropped and 50% effaced as of last week. Thanks!