I also didn't understand what was 陣痛 when I first felt it! However, I can tell you that 陣痛 may not be regular. If you feel very frequently (less than 10 minutes each time) that your lower stomach becomes hard, some back pain & feel like you want to go "ngng", then you may need to go to hospital lah.
My case was I felt 陣痛 at about 4-5pm on 1 Jan, then it became more & more frequently within 2 hours (3 mins, 5 mins or 7 mins), but not so regular as said in the books. I still didn't realise that I was about to deliver!!! Then at 8pm, when I saw red I went to hospital. Then at 10pm, I delivered my baby by c-section. My case is quite unusual, but pls take note.