The top of my left breast/arm pit muscle feels like it's Charlie horsing! What is causing this, and should I be concerned?

When implants are placed underneath the muscle the pressure may cause muscle spasms during the early postoperative period. As the muscle stretches out and becomes more relaxed muscle spasms normally subside. This may take a few weeks. Muscle relaxants can help with this problem. It is important for you to discuss this with your surgeon to make sure there isn't any other underlying problem.

Implants placed under the muscle can and frequently does cause muscle spasms at this post-operative stage.  The use of muscle relaxers under your surgeon's direction usually helps this.  It should resolve over several weeks, but you should discuss your concerns with your surgeon to make sure there is nothing else that needs to be addressed.