If your leaves were turning brown from the edges inwards, I would suggest not enough moisture as a cause but, if you are seeing lots of yellowing leaves, I would consider heat stress and check if you are watering too much. That can usually stress the plants and, if the soil stays wet for long periods of time, the plant cannot absorb oxygen; the leaves then turn yellow. So monitor the soil moisture by inserting a finger into the soil to a depth of 4" and give it 1 gallon of water (1.5g if the soil is sandy) but only if the soil feels dry or almost dry. To make the moisture last longer, maintain about 3-4" of mulch up to the drip line.

I am willing to bet that your plant is newly planted......if so this is a very common......happens every year at the nusery where I work
The plant is green and fine looking until it starts to produce flowers......the plant (being newly planted) cannot produce flowers without sacrificing some of its leaves....so it sheds the ones starting at the bottom......
You will find that this won't happen next year when the plant becomes established.....

I purchased this Little Lime plant last year when it was if full bloom, this is the first summer that it is growing in my garden from the very beginning. The soil does not appear to be damp below the soil line, although it's pretty hard to tell due to all the soil in my area feeling like cement from this heat wave. I will hold back on watering every other day or once a day, as I have been doing in this extremely hot weather just the same and see what happens.