Oo, perfectly normal yan sa 18 weeks! A few of us mums experienced the same thing around that time, ang uterus mo kasi ay lumalaki na, kaya parang may tightness or hardness na mararamdaman mo, lalo na pag active ka or nag-shift ka ng position. Isa sa mga kaibigan ko nag-worry din siya at akala niya may problema, pero sabi ng OB niya it's just the ligaments and muscles stretching to accommodate baby growth.
That said, kung parang too uncomfortable na siya or may cramping na grabe, spotting, or unusual symptoms, worth checking with your doctor or specialist lang para sa peace of mind, better safe than sorry, diba? Pero sa karamihan ng cases, it's totally a normal part ng pregnancy journey. Take care!