Pictures? To see how pink, etc.
Where are you located? Might help pin point a possible culprit.
What do you feed? (all feed, what you give and any pasture or hand grazing, weeds in the area) It could be from an imbalanced diet, something missing, too much of something.
Up to date on deworming and vaccines? What did you give? When?
Have you talked to your vet? [i know this should have been the first question, but its on the list.]
Have you recently sprayed for weeds or fertilized? Or any neighbors? Or even the city or county spraying ditches?