I have rib pain pretty much on one side, every day. It will be either in the front or in the back & its so uncomfortable. It generally bothers me the most in the evening & sometimes the only thing that helps is standing & stretching.
Like PP said, its pain from everything pushing on your ribs as the uterus grows. Some of your pain may be corrected by different sleeping positions but that doesn't seem to be the case in my situation :/