Sayang, this is quite common in second trimester actually, a few of us went through this too, but definitely mention it to your doctor at your next visit or call the clinic to check if it needs looking at sooner, especially if the pain is sharp or gets worse. It could be ligament stretching, the baby's position, or just how your body is shifting, but better safe than sorry lah when you're pregnant, so best to get it confirmed proper-proper.