Are muscle contractions normal after breast augmentation?

Please don't worry, muscle contractions on and off for up to six weeks after breast augmentation surgery when the implants are placed under the muscle is to be expected.  It is also normal for your muscles to clench down when you stretch when you sleep and will not cause any probelms.  In fact, stretching your muscles in a gentle way ultimately will help your recovery.

Not unusual when implants are placed under the muscle, especially in very active people. In my opinion implants should be placed above the muscle under the breast where they belong. 

Muscle spasm is common for the first few weeks. As the muscles stretch and swelling subsides this should improve.

Yes, this is absolutely common and frequent, if the implants are placed below the muscle.  I typically give my patients muscle relaxants routinely for this issue, and I have them stretch their arms over their heads to stretch these muscles out (in much the same way you stretch our a "charlie horse")  It initially hurts, but ultimately feels much better.  You should contact your surgeon to make sure all these ideas are okay with him/her.