Grab behind your nipples. If the lump is hard, it could be gyno. The only real way to find out (besides going to the doctor) is to lose a lot of weight and get into shape. It's very possible that the "lump" behind your nipples is just fat.
Also, if you are lactating, it's definitely gyno.
If it is gyno, I suggest taking Rebound Reloaded with Nolvadex and Lean Extreme for about a month (hit the gym/do cardio etc) and see what happens.
If it's fat, the Lean Extreme will shed some serious pounds. You could also try using cutting gel such as Lipoderm Ultra or Lipoderm Y. They release the fat molecules so they are able to be burned. Though, if you are not eating a calorie restricted diet and exercising, the fat can just come back to your chest (or go somewhere else).
With Rebound Reloaded - as long as you're on finasteride or dutasteride, you should be fine. I'd say combine Rebound, Lean Extreme & Lipoderm, hit the gym/cardio for a month and see what happens.