I tested positive for COVID on July 4th and got a negative result on July 13th. I'm still having issues though. I get really tired quickly, especially when going up stairs. I can manage to walk for about 20 minutes but anything beyond that and I start feeling exhausted and have this heavy sensation in my chest. I've had a chest X-ray, a D-dimer test, and CRP tests, and all came back normal. My SpO2 levels are consistently at 98. Should I be concerned about this? How long might it take before this weakness goes away?