What's your climate like right now? Pale leaves looks like a nutrient deficiency to me. That can be because you haven't fertilized it, or because of winter temps. Roots aren't efficient at absorbing nutrients <55 degrees F, so if your weather has chilled, this may be normal wintering behavior for your tree. You might do a search for "winter leaf drop," too.

ccarlson, have you been fertilizing this tree? The leaf symptom that the tree is showing is a nitrogen deficiency. (yellow leaves with green veins). Under sever cases the leaves become totally yellow. A citrus tree like yours should be fertilized 3 times per year. The deficiency in your tree's case looks rather server.