I have sharp pain in the left breast when I sit up, cough or move my arm. The right breast is pain free. Why is this? (Photo)

From
your photo, there is a small chance you could have a skin infection
(cellulitis), so I would see your plastic surgeon.That being said, this is a most common
complaint whereby one breast is causing all the problems while the other one is
pain free.When an infra-mammary
incision is made (incision under the breast), some surgeons place deep sutures
down to the ribs / chest wall to prevent the implant from ‘bottoming out’ …
these sutures can cause the symptoms you’re describing.Another possibility is that the nerves to the
breast, which generally come in from the side, are irritated from the
surgery.In both situations, however, I
find the pain will gradually subside, usually by 3-4 weeks after the surgery.

 It is normal to have some discomfort or pain in the postoperative period under the arm and along the lateral chest wall.   Your pictures make it a little difficult to  determine the color on the chest wall.    if there is no redness in the area then it is unlikely to be an infection. Other signs of infection would be drainage from the incision site, fever, chills, or general malaise. The area on the lateral chest wall where you having pain typically is an area which is dissected during surgery and may have some inflammation tenderness and swelling. Your best bet is to contact your plastic surgeon to discuss your concerns. Forums Such as RealSelf are no substitution for a good physical exam in office.

No two side heal alike. It is more common than not to have a bit more discomfort on one side as opposed to the other. This could simply be a muscle spasm (assuming your implants were place below the muscle). As long as you are not showing any signs of a bleed or infection-doubtful),  I would reassure you these normal pains will go away with time. If the pain is getting worse, contact your surgeon.

Thank you for your question. It is not uncommon to have a difference in the amount of swelling or discomfort on one side versus the other. While photos are helpful, they cannot replace the utility of an exam. I would contact your surgeon and let their office know your concerns. Hopefully, your surgeon will be able to address your concerns. Good luck.

It is certainly not uncommon to have pain when sitting up and coughing 4 days after breast augmentation.  The fact that your pain is on one side could be explained by the fact you're implant is a little more placed to the side than the other side.  There also is an asymmetry of your nipple areola which was present preop and probably required more dissection or a different type of dissection on that side which also could explain your pain.Even the you have a scheduled appointment Monday I suggest that you call your plastic surgeon's office and make sure your surgeon understands that you're having discomfort on one side.  The plastic surgeon understands the exact dissection that he performed and may want to examine you to rule out any problem.