I have an over-the muscle implant that was put in at time of mastectomy back in April this year. Mine is held in place with a titanium mesh, which has been stitched onto the muscle like a pouch.
It doesn't budge. It doesn't even really jiggle. The scar tissue near the armpit is quite thick and maybe contracted a little, so maybe that's part of why it's so firmly in place.
I did 'dent' it a couple of months ago (accidentally banged it into the corner of a cupboard door) which has created a little blob that can be pushed in (then pops back out). It's not visible to the eye but ultrasound can see it. It looks like I damaged the 'capsule' and/or stretched out the titanium mesh in that spot, allowing for the implant to bulge there. Ultrasound shows implant to be intact.
Hopefully your surgeon can tell you if what your feeling is normal or not. Thankfully an ultrasound can see the wall of an implant quite clearly if they are looking for a split or rupture (very unlikely).