Nak, please check with your OB agad, lalo na kung buong araw na siya at hindi ka comfortable, better safe than sorry talaga, especially at 21 weeks.
A few of us moms experienced heaviness and discomfort din during pregnancy, pero importante talaga malaman kung ano exactly ang cause para ma-rule out kung may issue. Your doctor will know best what's normal stretching vs. something na kailangan ng attention. Abangan mo din kung may fever o bleeding, those would mean emergency room na. Take care of yourself, ha!