I had sub-muscular breast implants after my mastectomy, 6-months ago. My Left breast is making an egg-like form at the bottom. My Doctor says it could be fat?

It's hard to answer this w/o an in person consult. Having said that, is the issue on the mastectomy side ? Were both breasts removed and reconstructed ? 6 months is not all that long depending on what was done. My suggestion is to return to your surgeon for an in depth conversation.