I heard a fatwa, but I understood it differently from what was intended. Am I sinful for that?

If you erred in understanding, there is no sin upon you, for Allah Almighty says: "And there is no blame upon you for that in which you have erred, but [only for] what your hearts have intended," and His saying: "Our Lord, do not impose blame upon us if we forget or make a mistake." And Allah Almighty responded: "I have done so."

If you understood a fatwa incorrectly and then the correct understanding became clear to you, then act upon it and correct the understanding for whomever you conveyed the incorrect understanding to, in order to clear yourself.