Often when tissue expanders are placed, the lower aspect is covered with Alloderm or other material. This is sutured in placed along the inframammary fold along the bottom, and to the lower border of the pec muscle at the top. It is possible that the implants are putting stretch on this suture line causing the pain, and I would expect this to get better with time. However, I cannot say this is the cause for your pain since I don't know the technique used by your surgeon. I would discuss your concerns with the surgeon who did your procedure, since he/she knows exactly what was done and the most likely reason for your pain.
Web reference: http://www.anitapatelmd.com/gallery/breast/