It sounds like you have been experiencing a number of unusual and bothersome symptoms recently. Any time you notice a change in your health or worrisome symptoms like this, the best place to start is with your general doctor
. He or she can talk with you in more detail about your symptoms and put this together with any prior medical history you have and any medications you might be taking on a regular basis. It will also be very important to do a thorough physical exam to see if there are any clues that might help suggest an explanation for this. In particular, it would be very helpful to see if your weight has changed recently or dropped off your expected growth curve.
There are many different things that can cause abdominal pain and nausea in someone your age. You could have an infection or even an ulcer (although that would be much less likely in someone your age). An esophageal web or stricture can cause difficulty with swallowing, and esophageal spasm can give you pain in the chest or upper abdomen. A condition called a hiatal hernia
(when part of the stomach starts to move up towards the lungs through a tiny hole in the diaphragm) can also cause symptoms similar to what you describe. To fully evaluate your situation, the best place to start is with your physician.