My best guess is cold, also. If lilies are rapidly growing, they are a lot more vulnerable, and it seems that if the duration and intensity of the cold is at that sweet spot where damage occurs but cell integrity is not compromised, leaves don't die or turn to mush, but other things might happen. This was of interest to me this year:
It is a Liliium distichum seedling that had overwintered inside an unheated garage. Already sprouting when it came outside, then 5 days later exposed to 30F. That pic taken four days after. The white showed up on the second day, increased in size for a day, then was static, but continued growth as the rest of the leaf did. Now, a month later, there is no change, but the leaves are larger:
So if the same thing happened to you, Mary, you will just have some permanently white leaves this year, and regular growth again next season. Not sure if this will affect further growth of the stem this year, or not.