Answer: It sounds like shock. Moving plants indoors from outdoors, or from a nursery/greenhouse situation can be a real shock to their system because the conditions are usually so different. Yellowing and dropping leaves is a typical reaction. Don't fertilize for a month or so, but continue watering slowly and deeply. The plant might go dormant for a while and then new growth should appear in the spring.
Citrus generally need 8-10 hours of full sun, so try to give more direct light. Citrus are also heavy users of nitrogen, so start fertilizing when your plant recovers. Try to find a fertilizer specifically for citrus.
In spring, slowly acclimate your plant to moving back outdoors by putting it outside in a sheltered location for a couple hours at a time, gradually increasing the time period. In fall, do that in reverse, rather than abruptly moving from outside to inside, which will help lessen the shock. Good luck!
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