Is It Possible to Have Torn an Internal Stitch from my Breast Implants?

You could have broken one of the deeper stitches but after a couple of weeks, this is not as concerning, especially if the skin is still closed and you do not have any signs of implant exposure (opening in your skin, seeing the implant, pain) or infection (fever, redness, tenderness, drainage).  Good luck!

It is possible that a suture internally may break but it is unlikely to result in improved arm movement since there should be no suture used that would effect arm movement. You should return to your surgeon and discuss this. Some things are not explainable but is there is no verifiable problem then it is not something to worry about.