Ay gemeña, that heaviness you're feeling is quite common at 16 weeks, lalo na if baby's sitting low, a few of us mums experienced that din and it turned out normal lang, pero since your OB already mentioned it, definitely follow up with her at the next visit para ma-monitor talaga kung ano ang exact position ng baby, kasi alam mo naman how reassuring it is to have the doctor check regularly. In the meantime, try wearing a maternity support belt kung comfortable sa inyo, and rest as much as you can, mahirap ang heaviness na yan lalo na pag marami kang ginagawa sa bahay. Keep your OB updated on any changes, ha, kasi yan ang safest approach.