It's very possible to feel the effects of finasteride within a little as eight hours after your FIRST application. And that means, you could start to feel pain in your sexual organs, or sensitivity in your nipples.
But there's no way gyno would manifest itself in such a short time. There's no way that much fat or tissue could accumulate that quickly in the normal healthy person. So, if your nipples have gotten puffy, it could possibly be water retention.
I would also imagine the accumulation of fat is a lot more reversible than tissue accumulation. That might require surgery, in the form of liposuction, or a partial mastectomy -- where excess breast tissue is removed. But again, if you're that far gone, I'm fairly certain it would be extremely EVIDENT.