All in all your tree looks under fed. However the leaves that the tree are discarding are older leaves. Further, because the tree is currently going through a flowers cycle, it is very common for the tree to discard leave. The flowering cycle (and flushing cycle) are the greatest time of the year that a citrus tree discards leaves.

Try adding some Epsom Salts to your water...2 1/2 tbsp to 5 gallons of water is what I read (I think...you may want to google.) I added some to mine and it looks as good as new :)

I agree with the not fed enough thought. Plants will often steal the nutrients from the older leaves to support the new growth and flowers, but it's not good for the long term health of the plant. What brand did you use?

It would be better to fertilize on a regular schedule rather than as a occasional quick fix. Osmocote Plus is easy to get and works great for container citrus. Many folks here also supplement with DynaGro Foliage plus. Don't use fertilizer stakes or granular fertilizer, that's more for in-ground plants.

I don't use epsom salts (magnesium sulfate)by themselves as it's easy to over or under dose with them. Too much can be a problem. Most quality citrus foods have Magnesium as a micro-nutrient in the correct amount. Also, to me it looks more like a nitrogen deficiency.

Magnesium deficiency and nitrogen deficiency both show up on the oldest or bottom leaves. Magnesium deficiency causes interveinal chlorosis (the veins stay green; the areas between turn yellow). Nitrogen deficiency causes the entire leaf to yellow. Your photo isn't great, but appears to show nitrogen deficiency.

How regularly are you watering? What type of soil is it planted in?