Any ideas what might have caused the below on my St Theresa Lemon?

It was wintered in my garage under grow lights at about 55 degrees. Toward the end of the winter, it didn't get enough water, wilted and dropped most of its leaves. Now seems to be doing ok, but thought I would ask the experts. I got worried and fertilized yesterday with Foliage Pro.

Thanks!

Spider mites

Thanks Laaz...

You posted a solution in less than 20 minutes.

Impressive!!

Now on to Home Depot to get some insecticidal soap... or is there something that works better?

When things are dry, spider mites attack. A dose of Bayer advance & keep your citrus hydrated & you shouldn't have a problem. Mix dish detergent with water & put it in a old spray bottle & mist the tree for a week or two.