I had same worry first time I changed the bag at home on my own and the bowel starting contracting and thought whole thing was going to disappear inside. My loop also sat very close to the skin and not very projected and did contract slightly so that instead of most of output coming out of stoma it was continuing through bowel as normal. I had another op 6 months later when I was having to have bm just as much as before op and so they did an end ileo instead which stopped the problem. The problem was my colon was so stuck together it was pulling on the stoma, so not a normal complication.
The stoma will change a lot after the surgery and I would just wait until your appt with stoma nurses and don't worry in the meantime about it disappearing - it won't. I thought the same and it never happened, even sat there not daring to breathe when it happened that first time as was too scared to move for 30 minutes.
Crohn's dx 1989, loop ileo 1992, end ileo 1992. Arthritis developed 1990 Stoma abcess 1995. Azathioprine started 1995. Panprocolorectomy 1999. Stillbirth 2000, antiphospholipid syndrome dx 2000. Flare up 2004, stoma abcess and strictoplasty 2004. Low blood pressure dx with impaired kidney function 2006, fludrocortisone pres. flare up 2010, steroids, Humira, codeine.