Q.Kaffier Lime Bush
I keep my plant indoors, in front of a sunny window. However the leaves are dropping off and not sure why. I don’t think I’m over watering. Is this common? Many thanks

Some interior and lower canopy leaf drop is normal as new growth appears. If you think it is excessive there may be a soil or water management issue.
If you think the light exposure is adequate and you have the water management right, not too much or too little, the next thing to consider is soil fertility.
How long has it been in the existing pot and potting soil? If a couple of years or more it may be time to re-pot with a good quality potting soil.
Do you fertilize? A good quality complete organic fertilizer can help supply plant nutrients to supplement what the soil provides.
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-moisture-in-plants.htm
https://www.planetnatural.com/potting-mix-recipes/
https://www.downtoearthfertilizer.com/products/blended_fertilizer/citrus-mix-6-3-3/