Dent in breast, close to sternum 3 1/2 months after BA. (photos)

Your complication is very common and clear, a well known one, as well as its correction.That dent is the margin edge of the pectoralis muscle, which normally is not visible but in your skinny frame and strong sportive muscle happens to be visible, and will worsen the next month as swelling subsides or even appear as well on the right side.Solutions suggested to you show poor understanding of the problem. Things are very very simple: conversion to subfascial plane: highly recommended 5th (latest) generation implants which are macrotextured, cohesive gel-filled and anatomical or teardrop-shaped, my preference is the modern, less aggressive and safer subfascial plane of location which prevents any kind of animation or flex deformity and muscular visibility.The excessive mobility is due to poor adhesion of implants to the
ribcage, which model are they? may them be smooth or microtextured
shells?
The modern shells have to be macrotextured, of high adhesion.
If the highest safety is sought then I'd doubtlessly opt for the polyurethane coated implants, they form a firm biovelcro and could help you in preventing your excessive mobility.If you wish better grounded opinion please do post or send privately well lit, focused and standard images: frontal, both lateral and both oblique views.
Feel free to request any additional information from me.

Thanks for your question! Judging by your photos, it does appear that you have "bottomed out" a bit. The only way to correct that is by surgically repairing the breast implant capsule pocket (capsulorrhaphy), & possibly with an implant exchange (I would go with a higher profile to give you more fullness up top, we call it "upper pole fullness"). 
One possible easy for for the "dent" would be to place a small amount (10-20cc, about half a shot glass full) of fat to fill that space in. Talk to your plastic surgeon about it: it's an easy revision that can be done under local anesthesia in the office by taking a little fat from your hip or abdomen & grafting it to that dent. Best of luck to you! 

"Bottoming Out" is the term given to implant #displacement, where the implant drops down below the existing inframammary fold; the natural crease beneath your breast. This may occur when the fold has been released excessively during surgery or may be due to factors of the patient’s collagen and tissue integrity. The implants then sit very low on the chest with a lack of internal support for the implant itself.Correction usually involves #restoring the crease beneath your breast to it's normal position with internal reconstruction of the capsule around the implant (#capsulorrhaphy). Sometimes a biologic fabric (Acellular Dermal Matrix or #ADM) can be used to attach to the breast tissue internally while supporting the implant.This corrects the placement of the implant, redefines the breast shape, and also refines the internal fold. Your board-certified plastic surgeon will discuss which approach is best suitable for you based your current implants.Typically, it's best to wait 6  to 12 months depending on the reason for #revision. Additional elective cosmetic #surgery is a personal choice. Your procedures should always be performed by a #Plastic Surgeon who is board-certified and has a great deal of experience specializing in #breast #augmentation #surgery and revisions.  Also, communication  and asking questions of your surgeon and the staff are key.