Does anyone know why symptoms of CFS and Lupus are so much alike?

These are my symptoms:

fevers around 99.9 up to over 100
extreme fatigue (which is probably related to low blood volume)
raynaud's syndrome
rash on face
sun sensitivity
low white count
ANA always positive
pattern always changes
red count borderline

Now these are all CFS as well as Lupus symptoms. How do you ever know what you have?