You may be full, maybe you pulled a muscle, or this is the result of exercising. Maybe you are nervous or stressed, it can cause your muscles to tighten. Also trapped gas can cause this. And ya know whats interesting, our bodies can have minor glitches like that all the time where you have an unusual pain or any other type of problem with your body and your not sure why. It's unexplainable but happens to us time to time. Don't worry I'm sure you're gonna be fine!
When your upper stomach is tight, it can actually mean that you have digestive disorders. It can also mean that your stomach is too full. You can lie down on your stomach and make sure that you see a doctor.