How can I tell when I'm having a real contraction?

I've experienced Braxton hicks before but today I did nipple stimulation and I've had two strong contractions while shopping 10mins apart but then they stopped when I went poop. Now sitting here a few hours later, my stomach keeps tightening and radiating to my back about 5min apart. I'm not exactly sure how to time them or even tell when the contraction is starting. I am so small that the tighting that I feel almost feels like baby pushing really hard. my entire stomach does get hard. the pains actually radiate into my legs a few times. please respond