Hi,
From what you are describing, this could be an infection or a condition called Gynecomastia.This problem manifests as a swelling of the breast or a lump under the nipple, including occasional nipple discharge when squeezed. The condition can last for a while, but will eventually disappear in a few months without any treatment.
The reason for this is "hormonal changes" in young males that often occur due to puberty and nothing to do with breast cancer.
Please talk to your mom or dad to take you to your family doctor just in case you have an infection that would need to be treated with Antibiotics. However, if it is Gynecomastia, the Doctor will be familiar with this condition from experience and can reassure you that there is nothing wrong with your breast. A lot of young men your age have this problem and you have nothing to worry about okay?
I would advise you NOT to squeeze your nipple, because as long as you keep doing this, the discharge will continue.
Take care...
Well, over the Internet it's hard to say if you have an infection or not. Only your doctor can tell you for sure what's going on, and if he/she thinks that it's an infection, then it should be quickly treated before it gets worse.
I still believe though that your problem is most probably "Gynecomastia" which is a nonthreatening condition and very common in teenagers going through puberty.
If you have to lose your left nipple a plastic surgeon could make you a new one closely resembling your remaining nipple PROVIDED that your remaining nipple is flat, and not the protuberant kind. I don't think surgeons have much success trying to duplicate a protuberant nipple but you might be able to false silicone nipple that would look OK.