Uy, normal lang yan mama, especially around 20 weeks, ang uterus mo is really stretching na at ang ligaments around it are tightening kaya sometimes feeling hard or tight ang belly, lalo na pag tumaas ka or nag-stand ka ng matagal. A few of us pregnant mums noticed the same thing, at ang OB namin sinabi it's usually Braxton Hicks or just the uterus expanding, pero para sure lang, mejor i-check mo sa doctor mo next visit para mapakita kung ano exactly ang nangyayari at para walang alalahanin ka. Drink lots of water, rest when you can, at kung sobrang sakit or bleeding, tawagan mo agad ang doctor ha!