I have a really bad pain on the left back side of my head and feel the left arm weak I have a brain injury do to combat what should I do now?

Seek treatment. You should seek treatment with your neurologist or your physiatrist.
Xrays of your neck. Your case sounds scary. Traumatic brain injury once treated and stable should not be the cause of your separate symptom; left neck/head pain and left arm weakness. Remember, left side of your brain, controls right side of your body. You might have injury to left spine/neck, perhaps from the trauma that injured your brain, causing left arm weak. Get appointment with your doc, neck x-rays, ct head.