Implants under the muscle but I can feel a definite edge of the implant on the cleavage side. Why?

Feeling an imant after breast augmentation is not an uncommon scenerio. Even though your surgeon mentioned that your implant is under te muscle, most of the time it actually is only partially under the muscle I what's called a dual plane approach. Most plastic surgeons will still refer to it as under the muscle to our patients. Therefore in thin people and larger implants, the implant aide and bottom is felt easily. 

You have to remember that what you have are implants, not natural tissues. They are synthetic and in the process of making a breast bigger can be felt as different than your natural breast. Palpability of implant edges are very common particularly in thin patients with little natural breast tissue and to the bottom and sides of the breasts where there is not complete muscular cover.

Depending on many factors, including the amount of breast and/or soft tissue a specific patient has, breast implants can be palpable in many areas on the chest wall, despite appropriate placement of the implants in the sub muscular position. Your plastic surgeon will be your best resource when it comes to ruling out a breast implant displacement or malposition problem. Best wishes.