Orange Tree, Leaf Drop, Curled Leaves, And Brown Spots On Leaves?

My indoor citrus tree is 2 years old and has been healthy and growing. No fruit, but lately there is leaf drop, brown spots on some leaves and curling leaves. Otherwise it looks healthy. Can yo tell me what could be wrong with it? The fertilizer I give it has been Miracle Grow, but I have cut back for the winter months. I water about every 2 weeks, when it gets fairly dry. Thank you!