First off let me say this - I in no way condone what you've done here. It's clearly evident that you didn't research what you were doing very well prior to use. This is your body you're playing with, and things of this nature can be very dangerous. I'm not here to flame you, but to offer some constuctive criticism, information, and advice. We've ALL made mistakes along the way in life guys, so let's help him and others learn from this instance in an effort to help prevent it.
Now, with that said - If your docotor seriously told you that he didn't think it could be gyno solely based on a unilateral presentation, then i'd say you need to find a new docotor. That information is not correct. While it is more common to have a bilateral presentation, a unilateral presentation is still very possible. If you have a firm and fairly mobile mass (not fixated to underlying tissue) behind your nipple that is sensitive to palpation, the odds are it's probably glandular tissue in nature. There is no way to be sure though without further diagnostic testing.
They'll probably want to do a diagnostic mammogram first (because the radiologists all want that prior to the Ultrasound.) The mammogram is basically looking for calcifications in the breast tissue (indicative of malignancy). If that is present, a diagnostic ultrasound is done to evaluate for bloodflow from a potential malignant source. They'll also do a complete hormonal profile (Total test, estradiol, LH, FSH, Prolactin, HCG tumor marker) and a testicular ultrasound. That's just the protocol for a complaint of this type in a male patient to rule out the potential etiology.
As far as the comment about breast cancer goes, it's a possibility(1 in every 100 cases is a male), but it's very rare in men. In those men that do have it, there is usually a very strong family history of breast cancer as well. It's more likely to be a non-malignant source, or cystic in nature (lipomas are common here.) I'd follow through at this point to err on the side of caution. If the ultrasound comes back normal, i'm sure that's as far as the eval will go.