Sayang, sakit macam period pain di bawah perut especially on one side at 31 weeks, kena go to your doctor or hospital check-up soon, ok? Jangan tunggu, better safe than sorry. It could be many things, tapi doctor kena assess properly. A few of my mum friends had similar pains mid-pregnancy and turned out ok, but some needed checking urgently, so jangan delay, go today or tomorrow! Keep track if the pain gets worse or if you ada bleeding or other symptoms, then go straight to A&E. Take care of yourself!