Why Do I Have Severe Pain From The Inside Of My Lower Right Chest, At The Right-side Bottom Of My Sternum?

Hi I am a 20 year old male, with hashimotos and mononucleosis. I have severe pain from the inside of my lower right chest, just right of the bottom of my sternum and approximately up 1 inch. I have no clue what it is and it comes on randomly at different times of the day. I have found lying on my right side helps alleviate the pain, while moving to my left side or on my back makes it worse. Any greater suggestionswould be great thanks!