Sometimes lumps in the breasts are completely normal and sometimes they are not. TeenHealthFX cannot tell you for sure what the lump behind your right nipple is without a face-to-face evaluation, so it is best for you to go see your doctor just to make sure everything is ok. Chances are, however, what you are experiencing is just a change associated with puberty.
It is unlikely that this lump is a blood cell because blood cells are extremely tiny and it would be almost impossible to feel one behind your nipple. Rather, it sounds like you might have a condition called "Gynecomastia," which simply means an enlargement of breast tissue, that is quite common in adolescent boys (approximately 60% of 14 year-old boys have some degree of breast development). Sometimes Gynecomastia consists of small lumps under one or both nipples, without any additional development of breast tissue. It occurs because during puberty both boys and girls are producing more sex hormones, including estrogen and progesterone, and in some males, their breast tissue may be temporarily more sensitive to the effects of those hormones. Gynecomastia occurs most often between the ages of 12 and 14 years and then goes away without any treatment. Even though you say that you have noticed that your lump is getting smaller, it is best that you see a physician soon, as male breast growth that has not stopped by 3 years might not go away without treatment.
Gynecomastia can also be a side effect of some medications or drugs, including antidepressants, certain prescription medications, heroin, marijuana, alcohol and anabolic steroids.
If you are otherwise growing and developing normally, have started puberty and now have this lump in your breast, it is very likely that you just have pubertal Gynecomastia and will need no further evaluation or treatment after you see your physician. But, since you do have a lump of unknown origin FX recommends that you get checked out by your doctor just to make sure everything is fine. He or she can let you know if anything is wrong and address any other concerns you might be having.