Probably not enough light. The warmer it is the more light necessary to keep leaves on the tree. They can be held as cold as 30s night and 50s day. That's what winters are like in CA orange groves central valleys. Works in my greenhouse but I do have good light so I could go warmer.

Winter brings less light for our plants. Supplement your light or accept the loss of some leaves. Also the cold may have done more damage before you brought your new? plant inside.

How much sun and what indoor temps has your tree been exposed to?

It is a common problem for potted citrus to loose leaves when moved indoors. The usual problem is the tree top continues to get sun but the root temps are too low to bring in water, so the leaves end up dropping. This can be a big problem for folks who keep the pot below the window level and tree up in the sun. IIRC the temp at which the roots stop working is ~55F, which is easy to get to if your plant is right next to a window in the winter.

There is quite a bit written out there on the web about this. Basically you need to get the roots and top in balance. Either less light to the top and/or warmer roots.