Ay, that heaviness could be so many things, ligament stretching, baby's position, or sometimes just how our bodies feel at this stage, kaya dapat talaga i-check mo with your OB to be sure! A few of us had similar sensations around that time, but every pregnancy is really different, so better safe than sorry, especially since you're already monitoring with doppler. How's your eating and water intake, sometimes dehydration or constipation (which is super common pregnancy-wise) contributes din sa that heavy feeling, pero don't rely on self-diagnosis lang, have your doc check it out properly, okay?