Oo, normal lang yan sa second trimester! I noticed the same thing around that time, parang biglang naging firm yung belly ko, walang sakit, parang ang ligaments ko lang nag-stretch habang lumalaki ang baby. A few of us from our mommy group experienced it din, kaya alam ko hindi lang ako, haha. But honestly, if it happens na madalas or may accompanying symptoms na weird, mas safe na i-mention mo sa OB mo during your next checkup para peace of mind lang talaga! How are you feeling otherwise?