Hello, I Recently Was Very Sick For 3 Weeks With

Hello,
I recently was very sick for 3 weeks with extreme fatigue, weight loss (20 lbs), naseau, diarrhea, abdominal pain, and trouble breathing. After CT scan, chest xray, blood work, and urine analysis, the doctors have not found anything. I am starting to feel much better but I am getting random moments in which my tricep and bicep muscles will get extremely "tired" feeling. I can still left heavy objects but they feel very fatigued. Any ideas what this may be? My glucose levles and thyroid appear to be fine as well.