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. 

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.
 

The body forms a natural barrier around the implants called a capsule.  It is highly unlikely that your implant has changed position.

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.

Your body formed a capsule of scar tissue around the implant during the first year after your augmentation.  There are factors that may have caused the muscle to slip over the top of the muscle during that period, but once the capsule is mature it will keep the implant in the same position even through breast feeding.  You should see a plastic surgeon who can examine your breasts and help you understand what is going on.  I would wait a month or two after you have stopped breast feeding.  The examination will be easier and you wouldn't  want surgery while you are nursing anyway.  Don't worry about it now.  Enjoy being a new mother.

As your breast augmentation was performed submuscularly six years ago, a well developed capsule has certainly formed around your implants. Nursing a baby will have no effect on the position of your implants however in some cases increased hardness may occur due to capsular contracture. Your implants Will not shift to a submammary position

After breast implants are placed under the muscle,your body fill form a "shell" of collagen and fibrous tissue called a capsule. The implant position will be fairly stable inside the capsule although it can stretch some due to gravity over time and occasionally the capsule may tighten (capsular contracture). During pregnancy and breastfeeding your breasts will enlarge and engorge with milk. These changes may make the implants feel differently but there is no reason to think the the implants will migrate from under to above the muscle. You may just be feeling the pressure of your enlarged breast on the implant.