Someone I know recently was very frustrated by muscle weakness and the only thing she and her doctor could figure out was that she had low sodium (131). Her muscle weakness had gradually come on to the point that she was unable to walk and went to the ER. She was hospitalized but in 5 days in the hospital no one could figure out why she was having the low sodium. After being home for a day or two, she went to a different ER but they couldn't figure out why her sodium was low either and all other tests were fine. When she was at home, on the floor, unable to walk, she thought to talk with a friend who was a clinical pharmacologist. He reviewed her meds with her and it turned out that a drug she was taking had the side effect of hyponatremia (low sodium). Common culprits are diuretics, antidepressants and pain meds as well as certain endocrine conditions. In her case it was an antidepressant. Her doctor is now weaning her off that drug. Her doctor has told her that she'll be back to normal, but it could take some time. I'd ask your doctor or nurse to review your medicines to see if any have the side effect of muscle weakness, lowering sodium or increasing potassium.
Another thing that might be causing muscle weakness could be your nutritional status. You say your BUN is low. How's your albumin? If your albumin is low (<4.0, especially if it is <3.5), talk with your dietitian about ways to increase it. Muscles need protein to function and low albumin puts people at risk of infection.
Exercise can help people increase their muscle strength, endurance, and flexibility. Exercise increases hemoglobin, improves mood, and helps to control blood pressure. You might want to look at the exercise booklet on the Life Options website (www.lifeoptions.org). It starts off very gradually so you can ease into it. A few minutes a day broken up into small chunks can help you feel better physically and emotionally.