My Husband Has Diabetes, Chronic Renal Failure But Not Yet

My husband has diabetes, chronic renal failure but not yet requiring dialysis, and MS. Within the past week, he's gotten progressively weaker. At times, he feels like he can't hardly catch his breath and feels weak and dizzy, but if he can get set down, he's okay. He also has chronic constipation and when he bears down to have a BM, he feels like he's going to pass out and has a spell where he almost cannot catch his breath. He had quarterly labs drawn for his upcoming nephrology appointment and his Cr was 3.55 and potassium was 5.something. What could be causing this sudden weakness.