Can an Implant Under the Muscle Move to over the Muscle After Nursing?

No, the implant can not move to another pocket with pregnancy, though over years the pocket can change to a degree. For the record, the pectorails muscle and breasts are approximately in the same area of the body except that there is no muscle in the lower outer portion of the breast so technically most but not all of the implant is below the muscle in a submuscular breast augmentation. 

It is difficult to say exactly what is going on without examining you but I have seen implants move out from underneath the pectoralis muscle.  This may occur if the implant pocket is dissected too low, if the breast tissue/skin moves inferiorly (for example after pregnancy) or following trauma.  It is also seen when implants are surgically moved from above to below the muscle (the implant may slip out of the sub-muscular pocket into the previously constructed sub-glandular pocket).

Try flexing your chest wall muscle and see if the implants react in a similar fashion (indicating that they are in the same location).  I hope this helps.

Implants can shift as the capsule (scar tissue) around the implant stretches.  Some of this is possible with pregnancy.  An implant can't really move from under the muscle to over the muscle in your kind of case...however, as only the upper 2/3 of your implant is covered by muscle normally, any downward movement of the implant can reduce the amount of implant that is technically covered by muscle.