Congratulations on beating the odds and surviving. I am responding as a fellow cancer survivor and not from the prospective of being a primary care physician. Any comments I write are not meant to establish a patient-physician relationship or provide medical intervention. All issues should be discussed with your personal physician.
I do feel that both Myofascial Pain Syndromes, fibromyalgia being one type, and chronic neuropathic pain syndromes may follow prior chemo or radiation treatments. Whether you have Fibromyalgia or another Myofacial Pain Syndrome is unknown. My own personal experience is with chronic neuropathic as well as myofascial pain stemming from both surgery and radiation nerve damageas a reult of my treatments. I have not personally taken chemo. When you look at Myofacial Pain Syndromes including Fibromyalgia your primary care physician has probably already considered other possible contributory causes like Hypothyroid, Polymyalgia Rheumatica, medication related like statins to lower cholesterol or anti-viral agents, cervical (neck) stenosis around the spinal cord or foramina where nerves exit the spine, other ongoing malignancies, tick borne disease like Lyme, chronic hepatitis C and other so called auto-immune disorders. Common interventions include consistent exercise (as tolerated), pain mitigating agents like Ultram, Neurontin, Elavil, Desyrel, Zolft, Zanaflex, Flexeril, Cymbalta, & magnesium + malic acid. Opiates like Lortab, Percocet etc and anti-inflammatory pain meds like Naprosyn, Motrin etc. are usually not recommended for Fibromyalgia but may be indicated for other Myofascial Pain Syndromes. There is no one agent fits best rule. You can even see physicians specializing in chronic pain these days but you should do your homework first and see if the one you are about to see has any real world experience with cancer patients.
Neuropathic pain is a very complex subject with host of causes and best dealt with a Neurologist with an interest in the area. Again, there are lots of treatment considerations from topical agents to oral meds.
In your head? Probably not. As a patient I flew down to MD Anderson in Houston, Texas for a second opinion concerning symptoms from extensive damage I incurred as a result of past twice day radiation therapy. While there is no current treatment for my nerve damage, I was most impressed by the professionalism, compassion and expertice I encountered. Second opinions for peace of mind sometimes are worth the effort.
Isolating oneself from others? Lots of possibilities here from completely benign (no big deal) to serious depression and a spectrum of other considerations in between. If you're not comfortable with the situation and worried about depression, hopelessness etc, you probably should make a trip to your PCP. If your comfortable/generally okay with isolation, then so be it.
I wish you many, many more healthful days.