What could cause numbness in 2 fingers on my left hand?

You're very welcome!

The two possibilities that occurred to me were a repetitive stress injury (inflammation inside the arm, shoulder, elbow or wrist can cause nerves to rub against one another and cause numbness and (usually) pain. Since you are right-handed, I wouldn't think that your illustration career would have anything to do with numbness in your left hand. Plus with RSI usually there IS pain that radiates from the point of injury either upward or downward, and most often that pain IS worse at night.

So that's what makes me wonder if you have a disc in your back that is getting squeezed, or herniated. Our vertabrae have gel-like discs between them, which keeps the bone from rubbing together and keeps our nerves from getting squished. When one of those discs gets herniated -- meaning it starts to get flattened and can almost slip out between the vertabrae -- the damage to the nerves can result in a pinched nerve, creating the numbness you describe in your ring finger and little finger.

Sometimes this can resolve by itself. In either case, however, you might be helped by anti-inflammatory medicine (either prescription or over-the-counter) and physical therapy. It may be that your second episode of back pain has been caused by your enthusiastic yard work, which has caused a disc to herniate and in turn caused that numbness.

Here's a good illustration and explanation of this:

http://www.spineuniverse.com/conditions/neck-pain/why-pinched-nerves-cause-back-neck-pain

Are you considering asking a doctor about this? It might be that with a few weeks of physical therapy and some exercises that you could say goodbye to both the back pain and the numbness.