Thanks for using the forum. I am happy to address your questions, and my answer will be based on the information you provided here. Please make sure you recognize that this forum is for educational purposes only, and it does not substitute for a formal office visit with your doctor.
Without the ability to examine you and obtain a history, I can not tell you what the exact cause of your symptoms is. However I will try to provide you with some useful information.
Weakness on one side of the body, involving the arm and leg, could be due to a problem of the brain, of which there are many possibilities, such as tumor and stroke. As you mention, a problem in the cervical spine (the upper part of the spine, around the neck area) can also lead to one-sided weakness. In cases of one-sided weakness, MRI of the brain should be done with and without contrast (dye) before a brain problem is definitively excluded. MRI of the cervical spine should also be considered depending on the history and examination. Some disorders, such as dystonia, can lead to dysfunction of the arm and leg without any abnormalities being found on MRI.
If a problem of the central nervous system (brain and spine) can not be found based on physical examination and testing, another possibility is a problem with the peripheral nervous system. This most often affects both sides of the body, and is distal to proximal (feet and hands before upper arms and legs), though exceptions can occur. Peripheral nervous system problems are suspected based on specific examination findings and history, and can be tested for using a study called EMG/NCS which tests how nerves transmit and how muscles respond.
Continued follow-up with your neurologist is recommended.
Thank you for this opportunity to answer your questions, I hope you find the information I have provided useful, good luck.
One of my daughters had severe problems with her nervous system that were never diagnosed. I told tthe doctors (and there were lots) I thought her problem had to do with a minor accident but was pooh poohed. I finally took her to a very famous chiropractor who located problems in her neck and hip. He explained that the minor dislocations were pressing on her nerves. In less than three weeks she was well on her way to recovery. She had been a semi-invalid from ages 8 to 12.
So try a chiropractor, but make sure he is a good one.