It does happen sometimes. Make sure that your case is expanding sufficiently in the expander die, but not too much as this will weaken your cases. .45ACP tends to not stretch too much, so normally you don't need to trim your cases. Also, the .45ACP headspaces on the case mouth, so be careful to not crimp too tightly. Another thing to watch is to make sure your bullet is centered and square to the case. Sometimes, if the bullet is canted slightly, it will bulge the case as you describe.
Good luck and happy shooting!
"I was home... What happened? What the Hell Happened?" - MM1 Jacob Holman, USS San Pablo