I'm glad your doctor was gentle and the shot did not hurt too much, my experience was very different but the result was the same, short term relief only.
I had PF in only one foot for quite a while, then for no reason it popped up in the other foot and now seems to be gone again. I suspect that if you have one foot more susceptible due to structural differences than you might just have it in that foot, I know mine is probably that since the wear patterns on the heels of my shoes are not symetrical.
I wouldn't advise another shot, most knowledgeable medical persons will only do it two times, maybe three. Over repeated use cortizone can eat away at the tissue it is injected into, especially the heel pad itself, which could leave you in much worse shape than if you had done nothing at all, a high price for a week or two of relief.