Ay, third trimester shoulder pain is so common, sis! A few of us moms experienced that, usually it's from sleeping position or the extra weight pulling on one side, so I'd definitely mention it to your OB just to be safe, especially since it started suddenly after you woke up. In the meantime, try switching up your sleeping position (pillow between your knees and under the bump helps), and honestly, a warm compress or gentle massage on that shoulder gives relief, one of my mum friends swore by just resting it for a day or two and it went away.