thank you for the additional information
Now the back pain, buttocks and groin pain when considered in light of the numbness that you can have can be an indication of a problem with your lower back. this could be in the form of arthritis of the spine or even a disc prolapse in the spine. This can also cause burning, weakness or even paralysis in the legs. The diagnosis can be made with the help of a clinical examination alongwith xrays and MRI of the lower back. The treatment can be with painkillers or even steroid injections. Severe cases may need surgery.
The weakness and lightheadedness alongwith weight loss point more towards a systemic disorder. You should be investigated for hormonal issues. Addisons disease is a condition in which you would feel weak and lightheaded alongwith having a low blood glucose level and low blood pressure. Specific tests to look for cortisol levels in the blood would be diagnostic. Another cause can be a hidden malignancy. You should have a CT scan of the abdomen and pelvis done, which should be preceded by a complete history and clinical examination to narrow down the site of the possible malignancy. If you havent gotten a ESR done, you should do so now.
I would STRONGLY suggest that you go to an internist and have him consider autoimmune disorders, hormonal issues including thyroid problems and malignancy as a cause for your weakness whereas the pain would have to be evaluated by an orthopedic surgeon.
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