Thanks for your query,based on the facts that you have posted it appears that you have Gynaecomastia
on left side which is tender due to super added infection.
This is due to hormonal
imbalance which is common at the age of adolescence
Please take broad spectrum antibiotics like Cefotaxime
along with anti inflammatory medication like Diclofenac
twice daily .
Do not worry this gets cured itself over a period of time and may take 6 months .