Hi - Without a photo it's difficult to tell you. I regularly fat transfer my post-partum patients whose nipples are in the correct position but whose breasts need volume after being deflated after pregnancy. If the skin is too stretched between the fold of the breast and the nipple, or if the nipple is just too low, then the skin will need to be reshaped before restoring volume, be it with fat or an implant. The best course is to see a board certified plastic surgeon to discuss your goals in the context of what your anatomy will allow. Be sure to ask lots of questions particularly about possible complications and the limitations of fat grafting. Best of luck to you!
If you have a small difference between the two breasts, then a fat transfer could be a good options. If you have at least one cup or larger difference between the breasts, you may need implants. Fat transfer will also not provide a lift. I would suggest meeting with a board certified plastic surgeon and discussing all your options.