Without pictures, it's difficult to say for sure what is going on. It sounds as if muscle contraction is causing significant change in the appearance of the implant. Unless extensive muscle release was performed by your surgeon, the implant should not be slipping in front of the muscle. Best to follow up with them--they know exactly what was done at surgery and can advise you accordingly. Best of luck!
I think it's best that you see your PS in person to determine what is happening. Without examining you in person, it's impossible to really tell what's happening.