Have you discussed it with your stylist? They might have some good recommendations.
If you are using any at-home coloring kits, you should know that they are much more harsh than what is used in a salon. That could be the cause.
Seeing as how you are hanging out on a fitness forum, I can assume you have a healthy diet, but I will throw that suggestion out there anyway.
You are what you eat!
And if all else fails, talk to your doctor. It could be a medical condition.