Lemon Tree

The leaves are probably comig off due to WLD ( winter leaf drop ), nearly everyone gets it with citrus, unless you keep the roots warm it will happen.

You need to measure the temperature of the compost in the pot, if its cold put it out of any direct light, the roots like to be at 70F to 85F , under 70 you will loose leaves if any direct light lands on it, if you loose leaves they will grow back when it warms up.

The best thing to do is to get a small heater mat, or pet bed heater and keep the compost warm, then it will grow and can sit in direct light,

When i have had no space on my heater mats for a new tree, i have had no problems with leaf drop by heating the compost up by standing it on a radiator till a thermomiter i keep in the pot registers 75-80 near the middle (it will be warmer at the bottom of the pot, but upto 110F or so will do no harm), when it is hot i leave it the rest of the day till i am going to bed and then heat it again, the cool in between times dosnt seem to matter, but constant warm is better

Room temperature should not be a problem unless its freezing cold, they like it to be lower than 12C or so for 600 hours a year in the winter to make them flower in the spring/summer, this is air temperature, not the roots

Humidity in the air should be as high as possible, preferably a humidifyer in the room, or stand it on a tray of water with pebbles in ( to hold pot off the water ) to make the air damp

Never ever water it unless it has dryed right out to about 2 or 3 inches down (like dust dry) , then soak the pot till it is saturated by lowering into a bucket of water containg a citrus plant fertiliser ( chempak citrus or similar ), never alow the plant to stand in a saucer of water, they are very sensitive to standing in water.

misting the leaves with a spray to raise humidity can be used ( is used by some, but has been proved not to do any good!) , i do it with dilute fertiliser once a week as fertiliser through the leaves works well

The reason the fruit is still growing is that if a plant is unhappy about something, even just a change in conditions, it will try to grow fruit to save the species

That's a great help. Many thanks for explaining.