This is my second year with chickens and last week the temp dropped to -17 degrees. We haven't had such low temps before and i had read to put petroleum jelly on the combs and waddles of those birds with large ones to prevent frost bite. i did this when i heard the temps would get below 30 degrees. the temps raised back up by the end of last week, but i noticed 2 days ago that one of my white Leghorns had yellow in her comb. is this something i should worry about? the other hen still has a healthy red comb and all the hens in my flock are acting healthy, including this hen. any ideas?