It’s not uncommon for citrus trees to drop leaves and buds when brought indoors for the winter due to low light, reduced humidity, and possible insect pests, etc. From your photos we cannot say if you are dealing with insect pests.  It may be environmental issues and it is best to focus on proper care.
The plant appreciates supplemental lighting during the winter at least 14-16 hours a day. Provide at least 4 hours a day of direct sunlight. Place outdoors in bright light during the summer. They like moderate watering, allowing the top inch of the mixture to dry out between waterings. Normal room temperatures are suitable. To increase humidity stand on trays of damp pebbles and mist spray occasionally. Fertilize with a high potash, tomato type fertilizer every two weeks during the active growth period only. Make sure the container has holes for good drainage.
See attached link for more information
http://extension.oregonstate.edu/gardening/meyer-lemons-can-be-grown-pnw-if-protected-winter
 mh