Ay, alam ko yan! Common siya, lalo na kapag lumaki na ang uterus, pero since you're only at 16-17 weeks, better to mention it sa OB mo next checkup para ma-check if it's just Braxton Hicks o normal tightening, or something else na kailangan bantayan. A few of us mums namin noticed the same thing, kaya don't worry too much, pero it's always better to be safe and have the doctor assess it directly!