Is the burning and stretching sensation just the muscle stretching and getting used to my implant?

Often, burning sensation is due to the nerves being stretched/regenerating. Best to give your body adequate time to heal since you are still very early in your healing phase. If you are still worried, best to call your operating surgeon so you can be properly evaluated in person. 

Although it can be uncomfortable and disconcerting, a burning or tingling sensation after breast augmentation can be an indication that your sensory nerves weren’t cut or damaged during the procedure. Your nerves were probably stretched and are now regenerating and sending intermittent electric shocks to reconnect with tissue, which causes the burning sensation. The burning should go away in 2 to 6 months as your body heals and adjust to the breast implants. If it fits with your plastic surgeon's post-op directions, you may want to try gently massaging your breasts in a circular motion to distract from the burning and help the nerves heal sooner.

The most likely cause of paresthesias(burning, itching, pins and needles) in the first weeks after breast implant surgery is the stretching or tension on the chest nerves.  The nerves which supply sensation to the nipple come from the side of the breast and pass just above the outside edge of the implant.  As the nerve gets used to the tension and the capsule forms the strange sensations fade away.  In your particular case if you are concerned about an infection or other cause you shuld contact your surgeon.