It happened to me too. mine was the right side and it started in week 20. It's basically baby pushing against your ribs and your intestines are being squashed. seen doctors regarding this but the advice is use hot relief pack while lying down. I applied it after work when I reach home. It helped a lot the pain disappeared in the late third semester. Not easy to sustain this pain while working. BTW, its not good if you avoid wearing bra, it causes the breast to sag as they are much bigger and heavier now and it may clog the milk ducts and hinders breast feeding. I wore underwired bras and sometimes too loose, the milk ducts are consistently clogged, eventually I got breast infection and my girl could only enjoy a few days of breastfeeding..