Aih, sakit rusuk masa pregnant memang common, tapi since you're feeling it quite often on one side je, better safe than sorry, please check with your doctor or midwife at your next appointment, or if it gets worse, don't wait lah. A few of my mum friends had similar niggles in the second half of pregnancy and it turned out to be nothing serious, but your doctor needs to rule things out properly. Take care of yourself!