The yellowing leaves on your Meyer lemon could be one of several things. 1. Citrus mites commonly attack these plants indoors. Look under leaves for a dirty, webby appearance. The mites are too small to see. There are miticides available that are made from botanical extracts you can use, spraying it on the top and bottom of foliage. Be sure to apply before the plant comes back inside next year and at least once in mid-winter. 2. Lack of light – as winter goes on there is less light for citrus and these plants want lots of light. So if the leaves aren’t getting much because of the dark days the plant starts dropping leaves. If your leaf drop began in January this might be the reason. 3. Many plants start dropping older foliage at this time of year as they begin to put their resources toward the new growth. That might be what’s happening here.
Be sure that you’re keeping this plant evenly moist but not too wet. Be sure the pot still has good drainage – no rocks in the bottom of the pot and nothing covering the drainage hole! Be sure that you’re fertilizing regularly in that citrus are one plant that appreciates regular fertilizer. Use any fertilizer according to directions.