Q.Lime Tree
My lime tree produced lots of fruit in the summer. I brought it inside for the winter and it is now dropping leaves at a scary rate but is producing flowers. Not sure what the problem is. We do have underfloor heating and maintain a constant temperature approx 19-21 degrees. I water when the soil is just going dry and fertilize.

Certified GKH Gardening Expert
It may be suffering from shock. If a plant is not acclimated well when transferred indoors or outdoors, it can shock the plant. This article will help:
https://www.gardeningknowhow.com/houseplants/hpgen/acclimate-plants-indoors-winter.htm
I would also increase water. Don't swamp it, but don't let it dry out. Citrus like a little bit of humidity and many homes are rather dry in the winter, so the extra water will help.