This sounds normal for a lemon tree. When the plant is brought indoors they are dealing with less light, humidity, and change in temperatures. The plant has to reacclimate to these growing conditions. The tree will drop leaves and the flowers and lemon may drop off.  The tree will put out new growth in the spring. 

The tree looks tall and spindly and it will grow better as a shrub. In the spring prune the plant (top it) to about 3-4 feet and let the side shoots come out. Once you locate the tree outdoors after frost  hopefully it will flower in the summer and it will get pollinated outdoors. Fertilize only when actively growing in the spring and summer.

Now -  during the winter, a position near a South-facing window will provide the strongest light. Supplemental indoor lighting with a fluorescent or LED grow light will be necessary if ambient light is insufficient. Do not overwater. Check your plant’s soil moisture once or twice each week. Press a finger down 2” into the soil. If it feels dry, it is time to water. Thoroughly add water until it drains out from the bottom of the pot. Remove any excess water from the saucer or tray underneath; plants should not sit in standing water.  Here is how to grow dwarf citrus 

Marian