Your doctor's response to your telling him what is going on is more than disappointing. It makes absolutely no difference that he doesn't think it's cancer, and I do not think it is either. But he operated on you for cosmetic surgery and your problem and pain stems from the operation, (and even if not), and cancer was never even a concern, or otherwise he would never have done the operation in the first place. Also, breast cancer does not hurt like you describe your pain.
So, what your doctor needs to do is hear you and then treat the problem by either draining the fluid or removing the cyst just like japdip suggested, and not dismiss you for reasons that are totally irrelevant.
What matters here most is your pain and it's his responsibility to help you fix the problem.
Do not take what he said about this not being cancer as a 'no, it's nothing'.
call him back ASAP and make an appointment.
I am sorry you got treated this way.
KATRIN
Evidently your tests have shown that this is a cyst ... these can be drained or removed completely. You might consider making an appointment with a Breast Specialist or Breast Surgeon and take your films with you. He/She may have some additional advice concerning this problem. Regards ....