It could be one of a range of reasons, and possible more than one. Yellow leaves usually means it could be a nutrient deficiency of some sort. Possible culprits could be Iron, Magnesium or Nitrogen. Given that you can't be sure, a good fix would probably be to give a dose of a general purpose citrus food. You would then probably follow this up with another dose a couple of weeks later - but ensure you follow the directions.
Are you growing it in a pot? If you are, then you may need to look at often you are fertilising. Nutrients are often depleted out of potting mixes fairly quickly - every bit of water that drains out the bottom is leaching nutrients with it.