Lemon Tree leaves spotting up
I'm hoping some of the very knowledgeable folks on here can help me. I got a meyers lemon tree in February from Amazon as a gift. I potted it up, and for the last 6 months it seemed to be doing very well. It went from one little twig to a branched out shrub shape- lots of new growth and it's flowered a couple times. :) I'm currently letting two lemons grow on the older branches (they're a little bigger than a golf ball and still green). I love my tree and have been so happy with its progress!
Recently though, I noticed a few yellowish looking spots on the tops of the leaves. They look like transparent spots in the leaves because they only show up when the sun is shining through the tree branches. On the bottom side of the leaves are tiny little bumps in the leaf surface that seem to correspond with the yellow spots on top. They've spread to almost every leaf, and I don't know what to do. :( Please, can someone help me before more damage happens?
Here's the pertinent info:
-My tree is in a ceramic pot with Miracle Gro Cactus soil mix.
-It's in a south facing window with an east facing window next to it, and a grow bulb above it.
-I water it once every 7-10 days when the soil dries out.
-I use DynaGro Foliage Pro fertilizer each watering per the instructions on the package.
-In between watering I mist the leaves to keep the humidity levels up..
-I can't find any evidence of pest damage (other than the spots on the leaves).
What am I doing wrong for my poor tree?! Can someone please help?