Why are my boobs shrinking?

Your body will be undergoing many changes, much that cannot be appreciated with a simple question.  Your breast size will grow until the age of 21 most often.  Another aspect is to look at your grandmothers and mother to get an idea.

You are young, and your beasts are still changing.  Love yourself for who you are, not what your breasts are, and certainly not for what your boyfriend thinks you should be.  

At 15 years old your breast will continue to change and develop. Unfortunately there is nothing that you can do to increase your breast size. I recommend waiting until you are 17 to 18 years old, and if you are still unhappy, then you should schedule a consultation with a board certified plastic surgeon to discuss your options. 

Not much info to go on here.  You don't mention, for example, if there has been a weight change. The question you ask would best be asked of your personal physician. As to the second question, only surgical breast enlargement is effective.All the best.