Pace
I still get them occasionally, days when the body is feeling so bad that you want to give up. However, these will pass eventually. They triggered could be from medication to just health condition, from mood to environment. We are still human beings after all.
If you are going to visit a General Practitioner (GP), make sure you inform them of your pre-existing conditioners, meds you are on, and how are you "feeling" unwell. Rest and diet do play a part too. Sometimes your body just need you to break the routine and just take things easy for awhile so that you can continue to get better - like a rechargeable battery - it needs to be recharged. When and if you doing activities like exercises or such, take note to take things slowly when you are unwell. Perhaps even stop or skip the session for now. Consult a physician. In the cardiac rehab program, because of my lack of sleep for a few days, we found my blood pressure had gone up. So the physiotherapist had decided to halt half-way and consult a doctor. We are what we are when we feel unwell - everybody has them.
Break away for awhile - don't be stubborn about it. Come back later when you are feeling better.