Aiyoh, pain on the left side during pregnancy can be from so many things, gas, ligament stretching, or just your uterus growing, especially at night when you're lying down and everything feels more obvious lah. A few of my mum friends had similar complaints around that time and it turned out to be nothing serious, but since you're in your first trimester, best to call your doctor or clinic just to be safe and get their peace of mind, okay? They can check you properly and make sure everything is progressing normally, better than worrying at night!