A few questions. How big is it? How long have you had it? What size pot is it in? Does the pot have drainage holes? Where in your home is it sitting? How often are you watering it? Need a little back ground info to give you an informed opinion. Thanks, Donna
Four Seasons you may be right about the watering. My lemon tree did this a lot in the winter because I often forgot to water it. Combine this with the dry heat in the basement...The fruit drop could be a nutrient deficiency due to being in a pot and getting washed out when watering. See if a dose of Miracle Grow plant food works.
As Four Seasons and Flowerscapes have indicated, citrus grown indoors is subject to far more stresses than trees grown outside. The two chief things to remember are that, no matter where it is grown, citrus needs a regular supply of water, and that is a heavy feeder. Stick your finger down into the pot your tree is growing in. If it is dry 3 inches down, water deeply. Citrus needs more nitrogen than any other nutrient, so look for a slow-release fertilizer has at least a 2-1-1 ratio of nitrogen to potassium to phosphorus. The good news is that a young tree produces far more fruit than it can support, so you would probably have had to remove a lot of that fruit anyway.