I had something similar to this, still kinda do but it feels normal now, I was worried for ages that my left breast pocket was to big and that was why it sat to side and did same thing u describe when sleeping, it also seemed to move a lot more than the right. Anyway when I went to see surgeon at 3 months I explained my concerns and he said it’s was the way my boobs sat, when he showed me pics of before my op and to my astonishment he was right. I had never noticed before because my boobs were so small. It is not incredibly noticeable but when I lay down my right stay upright where as left falls to side a bit, I have more side boob with my left. I am now four months and they have settled in and I am use to the way they move and it no longer bothers me when sleeping etc x