I bet this pain must be kind of scary at times but it is very likely to not be a serious problem. Still, it will be very important to find the cause and treat it so that you won’t have to fear that the pain could come back anytime and, when it does, worry that it could be something serious. You should see your doctor who, after gathering information from you about the details of the pain and after physical exam of your chest, heart and lungs and a chest X-ray, will probably have a pretty good idea about the cause. If not, further testing may be necessary.
At your age pain in or near the sternum is most likely to be what we call musculoskeletal; that is one that originates in the bone, cartilage, ligaments, nerves or muscles of the chest wall, this being a pain that can occur with breathing or even when you hold your breath.
On the other hand the “sore throat and the pain with a yawn, burp, swallow or cough” are suggestive of spasm of the esophagus (the “food tube” that runs from your mouth to your stomach. It is a muscular tube and can hurt just like a muscle spasm in your leg. A common cause of such spasm is when stomach acid flows backward into the esophagus (acid reflux) X-rays of the esophagus might be necessary to further evaluate this possibility.
These are just some possibilities. There are others of course that need to be ruled-out (found to not exist) but many of these can be dismissed by a normal physical exam, chest X-ray and maybe an EKG if there is any consideration of irritation around the heart.
You need to see your doctor to find the cause, get relief of the pain and ease your mind. I note your mention that you “have never kissed anyone in my life” and that would sure reduce the likelihood of your having contracted an infectious disease from another person, such as Mononucleosis. Interestingly one need not have to have kissed someone to contract the disease and that disease, that causes lymph node enlargement, can sometimes be a cause of some of the symptoms you’ve experienced including sore throat and pain with swallowing and burping.
See your doctor and try not to worry about what’s been happening. There is a very good chance that you are going to be just fine.
Thank you so much!!!!! It turns out it wasn't anything serious just an acid reflex thing occurring because of my milk allergy. but thank you for being a great help!:)