Meyer Lemon house plant disease?

I've got a three year old meyer lemon house plant that has recently started to lose a lot of leaves. I moved it inside and some leaves started yellowing and falling off. I thought it wasn't getting enough light through my window so I bought a grow light and have that on during the day now. It is still dropping leaves, and they appear to be developing splotches where some of the green leaf pigment seems to be leaving areas of the plant. Eventually, the leaves start to develop yellow spots on the top and blackish spots on the bottom of the leaf. I've attached some pictures. Any ideas what's going on?

I don't know about Meyer lemons specifically.

When our indoor lemon and lime trees started losing a lot of leaves, the research I found said possible causes were BOTH too little water or too much water, which was extremely frustrating to say the least!

I started measuring how much water I give them, and also instituted 'water Wednesdays' - a specific day to water. As long as the watering is pretty regular, the trees don't drop a lot of leaves. I don't think they get as much sunlight as they really need, but until we have a greenhouse, it's the best we can do.

They're about 7 years old. The lemon has been giving us about a lemon a year for 4 years. This year we've just harvested one, and the tree has 3 others in various stages. The lime tree has set some tiny little fruit, but never kept them past that stage.

Best of luck!