Chronic Fatigue Syndrome isn’t apart of RSD. They aren’t related to each other in any way. This doesn’t mean that you couldn’t have CFS, though. The treatments for CFS often includes antidepressents, allergy medications and NSAID’s, like Ibuprofen and Aleve. Sometimes immune system boosters too because the illness is similar t0 a cold or flu. The experts can’t come up with an exact cause for it, but some believe multiple factors can be involved such as genetic factors, hormone abnormalities, and stress. No actual symptom of RSD and CFS are similar or linkable. But someone with one could share similarities with the other.
Talk with your doctor. Maybe they will be able to learn what is going on with you better and understand how to ease your pain and discomfort.
If you think disability is a better route for you, definitely apply. You can either file on your own behalf (and do all the tedious paperwork yourself including releasing you medical records) or hire a SSD attorney to assist you. Will will need your medical history and doctors to deem you disabled an unable to work.