Weight gain is one of the possible side effect of having Implanon.
Progesterone (Implanon is synthetic progesterone) stimulates the growth of tissue in the breast, so, you get the feeling of larger breasts.
Breast leakage can be due to use of birth control pills (I'm not sure for Implanon),using some other drugs or due to thyroid disease and benign tumor of pituitary gland.
Intense and continued stimulation of the breasts can also lead to leakage of milky fluid.
All of the mentioned possible causes, usually lead to breast leakage in both breasts.
When the fluid comes from just one breast, it is much more likely that the cause is localized to just that breast.
Have you already got your blood results?