"Like menstrual cramps"

People have told me, and I've read, that contractions (little ones, at least) are like really bad menstrual cramps. The wisdom usually goes like this, "oh, you'll know when you're having a contraction. It's like a really bad cramp when you have your period!" Here's the thing, people. I never had regular periods. That's where the whole "I didn't think I could get pregnant" thing came from. And the periods I did have were usually tame (and often times very short). No cramps. Maybe the occasional back ache, but that was about it. So all these "really bad cramp" analogies aren't doing it for me. Sure, at a certain point, I'll know I'm having a contracting. But in the meantime, I'm drawing a blank.