My blood work shows creatinine level quite low-.39. since sept, extremely weak in my muscles/fatigue easily/must rest often. what would cause this?

Many things: can cause your symptoms. However, a thorough evaluation of your thyroid levels, blood count, and current medical conditions could help sort things out. Follow up with your physician.

Many things: The low creatinine suggests that you have low muscle mass. It isn't clear if this is based on your being small or due to a concerning problem. Many things can cause this like a primary muscle problem, metabolic issues, hormonal issues, nutritional deficiency and low grade infections. You will be best served by reviewing your issues with a physician to better create a course of action.