Help! My Dwarf Meyer Lemon Tree is Losing Tons of Leaves?

I've had my 2-3 year old Dwarf Meyer Lemon tree since winter, and generally have had no problems. There was a brief incident with scale, but that is now under control. The tree has blossomed and now is starting to produce lemons. But about two weeks ago, it started losing leaves like crazy. There are several lemons-to-be on the tree, two about the size of nickels and they seem okay, although some tinier ones have fallen off. There is no apparent cause for the sudden balding. Any ideas? I live in Oregon. Through the cold/rainy season I keep the tree indoors with a special light to make the tree think he is in sunny southern California. I moved him outside earlier this week in an attempt to stop the loss of leaves. So far it seems to be helping. Today I lost only 2 leaves, whereas indoors he was losing around 6 a day. Maybe he just needed some humid outdoor air? I have the tree potted in a soil recommended by my local garden center, mixed with a bit of the shavings that the tree was mailed to me in, as the potting instructions suggested. I don't know how the soil could be the cause of this issue though, since the tree has been in the same mix since November and has done well up until now. Any ideas of what could be causing the loss of leaves? Or suggestions on how to make it stop? Thank you!