What sensation one has if internal stitches snap from the breast implant pocket? Does the outer skin become lumpy or swollen?

It really depends on the surgical techinque that your plastic surgeon has used. I use anchoring sutures in the infra mammary crease area to define the new crease and to ensure that your scar comes to lie in your new crease. If these sutures have been used, there is a possibility that they might have given away. However, if you do not have any discomfort and if it hasn't affected the shape of your breasts then I wouldn't worry about it. It is best to consult your PS who will be able to explain about any sutures he may have used on the inside and examine you to ensure that no damage has been done to reassure you. 

Yes, it is possible that you loosened a suture or perhaps scar tissue. It is difficult to say especially since I did not perform your surgery. Best to review with your surgeon.