Could getting a colectomy mean I'll have to get a colostomy bag?

No bag. A colostomy is performed when the bowel is resected and there is a significant length of proximal colon and minimal or no length of distal colon to bring the ends together as with an APR. When a stoma is present, irrigations should be started on a daily basis and a square pad placed over the stoma. With daily performance the colon will eventually function only when irrigation employed.