Potted Avocado Tree Loosing its Leaves?

I have around an 8 foot/2.4 meter avocado tree, which has been grown indoors in Colorado. The bottom leaves are turning brown and falling off slowly, but the top is growing well. The tree was grown from a seed from an avocado purchased at a market. It has only received fertilizer once, and it was planted, after roots formed from the seed, around 3 years ago. The soil is a good mixture with no rocks or pebbles on the bottom of the planter.

The tree was subjected to cold air for 8 hours last fall and has never been as healthy as it once was. The sliding glass door was left open near the tree.

I have so many questions! Should I try to re-pot it? What would you recommend for fertilizing and how often for such a tall tree? Is it normal for bottom leaves to fall off for a taller tree?

Many thanks!