Oo, normal yan sa pregnancy, lalo na pag lumalaki ang baby at nababago ang posture mo! A few of us sa mommy group namin experienced the same thing around that time, kaya don't worry, pero since you're already at 16 weeks, mas better kung check mo na with your OB para ma-rule out any other concerns at ma-guide ka properly on what stretches or exercises are safe for you. In the meantime, try to rest when you can, avoid heavy lifting, at maybe use a pregnancy pillow para sa side sleeping, ganyan ginagawa ng isa sa friends ko at tumutulong daw talaga.