Hi mama, that sounds like ligament stretching which is super common at 21 weeks, pero since you're a first-time mom and the cramping is worrying you, best to mention it to your OB just to be sure everything's okay, better safe than sorry, right? A few of us felt similar tightness around that stage, parang ang belly mo talaga nag-aadapt sa baby growth, pero your doctor should check if it's just that or something else to watch out for.