Occasional pain (tingling, shooting) is not unusual. Based on your description, you may have suture coming out on the right side. You should go see your plastic surgeon to be evaluated.
But if you are developing a contracture, you should see your doctor about what options you have. Cleavage in and of itself is not determined by whether your implant is above or below the muscle but more on how your pocket is created. Your inferior medial pockets seem a little...
The key word in your post is "saggy." You did not include a photo so let's go by that description. Implants alone will add volume to your breasts but they will not lift them. That requires a breast lift procedure. If you are saggy and you put implants under your chest muscle...