Hey mama, totally normal yan sa 16 weeks, ang uterus mo ay lumalaki na talaga kaya may firmness/tightness na nakakaramdam ka especially when you're lying down, a few of us felt the same thing din! But since nag-aalala ka talaga, I'd suggest giving your OB a quick call instead of waiting til Nov 27, hindi naman mahirap magtanong at peace of mind mo is important, plus they can check kung okay lang talaga or if it's just ligament stretching. Wishing you all the best with your pregnancy, mama!