Why does my stomach hurts when I stand up straight?

Hey, so I'm a 16 year old guy, I was having just an ordinary day when all of a sudden I stood up straight and my stomach just started to really hurt, it felt like it was tearing apart. So I slouched over and sat down and the pain kind of went away. I'm pretty sure it's not muscle tear cause I didn't do any ab work outs that day. I located the pain to be more in the left/center part of upper abdomen, I'm going to go see the doctor tomorrow cause a few days ago I had nausea for almost a whole week it recently got better yesterday and now this **** with my stomach. So i would just like to know if you guys have any valid opinions about what the hell i have. Thanks.