Is there a reason why I am getting constant pain on left side of head only?

Headaches. Are very specific to each person. If your doctor thinks there is a problem inside your head - your brain... Then get an MRI rather than a ct. The ct is lots of radiation. The MRI is none. The MRI is more detailed. It costs only 250-500 on insurance. And.. It is more definitive. Get it only under a doctors supervision.