Contractions

So everyone says "you'll know when they're contractions" "they feel like bad period cramps" okay well I was never one to have cramps and I've had a pretty easy going pregnancy (i dont know if it's cause of a high pain tolerance or what) but how do you know when to start timing contractions. For example. My baby moves. A LOT. In my ribs mainly. But tonight they build up to the point where it hurts. And they come and go and it's not just in one spot. So how do I know if it's real vs fake contractions or if it's just her moving differently? I'm 38+1