Ahhh, at 36 weeks tu memang common lah untuk rasa macam nak berak tapi takde hasil, biasanya sebab baby dah turun dan pressure kat bawah, bukan sembelit sebenarnya. The heaviness and that straining feeling you're describing could very well be the baby's position pressing down rather than actual constipation, walaupun lactulose tak really membantu. Some mums I know said they felt exactly that sensation weeks before labour, and some felt it on and off throughout, so it's not necessarily a sign labour is imminent, tapi definitely worth mentioning to your doctor or midwife just to be sure, especially since you're already at 36 weeks. They can check baby's position dan give you better guidance on what's happening. Take care, ya!