weakness in left arm below elbow for a week or so. Potential issues?

It could be any number of differant conditions.

Maybe related to the elbow, a disease, or Injury to the muscles or tendons connected to the bones in the lower arm.

Forearm strain is common because of the many tendons that glide together in the same or separate sheaths. Muscles, tendons and bones comprise units. These units stabilize the elbow and wrist joints and allow their motion. A strain or weakness occurs at the weakest part of a unit.

Check with a doctor for sure.