Lindsey, you might be overthinking this. Don't forget it's a little tree, and won't grow as lustily as a regular plant will. Btw, citrus do lose leaves all the time. They are evergreen so keep making new leaves and losing the old ones. As long as it has healthy leaves up top, you probably shouldn't worry about the loss of leaves lower down. The leaves I'm seeing all look good, shiny and a nice color. Just make sure it's getting maximum sun.
Did you pot up the plant yourself when you got it, or did it come in that pot? I'm just wondering if it might be worthwhile for you to slip it out of the pot gently and have a look at the roots. I would lay the pot on its side on some newspaper, and slide the whole root ball out. Take a picture and post it if you like. But if you detect a bad smell or see bugs or something, then some action might be in order.
Also, if you have a magnifying glass, take a close look at all the leaves and stems with it. Be sure to check the backs of the leaves as well as the trunk right down to soil level. There can be bugs that hide in plain sight, like scale insects that have a little shell over them and camp out on the stems. They just look like little bumps on the stem until you look closely. You can remove these with your fingernail if you see them. You're also looking for spider mites (although if you've been showering the plant regularly it's doubtful there would be any) or aphids which would appear on the tips of the stems, where the new growth is. I don't see any discolored leaves or signs of damage from insects, though. But I can't see as well as you can.
