I didn't notice weight gain from mine, unless it was from overeating due to depression at it not working and the doctor doing such a bad job that I was in more pain afterward than I had ever been before.
Anyway, from a single injection, the amount of cortisone (a steroid) used shouldn't actually be enough to cause long-term weight gain like you'd see if you were taking steroids on a daily basis. How long has it been since you had the injection?
3/10/14 Started Atkins @ 199 pounds.
7/18/12 became wheat- and dairy-free @ 235 pounds
Highest weight ever: 260