Mama, dahil may pain ka and you're 21 weeks preggy, mas safe talaga na i-check mo ito sa OB mo next visit or sooner kung worse na siya, better safe lang, di ba? Pain sa ribs at sides is pretty common sa pregnancy dahil lumalaki ang baby and shifting ang organs, but your doctor needs to make sure it's nothing else, lalo na kung first time mo to feel it. One of my mum friends had something similar and it was just positioning, pero importante pa rin na i-confirm ng doctor. Praying it's nothing serious, ate! Let me know pano na pag-check mo ha.