Has my breast started getting a double bubble or normal sagging? What can I do to fix it? (Photo)

This is a problem related secondary to likely too large of an implant exerting forces on your tissues over time. You will likely need to downsize to a smaller implant and use an internal support system such as Galaflex to provide support to your breast tissues. Depending on your size goals, you may also need a breast lift to correct reposition the soft tissues over the breast mound. I would recommend seeing a board-certified or board-eligible plastic surgeon with aesthetic surgery training and experience to tackle this type of problem in a safe and efficient manner. Best of luck!

Dear wiiinter,
I understand your concern. However, without a proper assessment, it would be difficult to determine what went wrong. It is best that you visit your plastic surgeon for further assessment or ask for a second opinion. Only after a thorough examination, you can get proper recommendations and advice.
Daniel Barrett, MD
Certified, American Board of Plastic Surgery
Member, American Society of Plastic Surgery
Member, American Society of Aesthetic Plastic Surgery

You may have capsular contracture. Go visit your surgeon or get a second opinion from a revision breast surgery expert.

You are bottoming out on your one side. This can be fixed by tightening the inferior pocket to help raise up the implant.

Based on the excellent posted photos you appear as a very good result. Remember the results are NOT twins but sisters. Best to return to see your initial plastic surgeon for follow up examination.