I had a ct scan a couple years ago and the contrast came out of my vein and into my arm instead. ever since my whole inside of my arm where it happened all the way up into my chest all you can see are big green veins. its quite scary. why is this?

I doubt those veins are caused by extravasation of iodine contrast. By Radiopaedia: most contrast media extravasation results in minimal swelling or erythema and has no long-term sequelae. A large study found above 97% of patients with contrast extravasation had minimal or no injury and reported that 79% of patients had localized swelling after extravasation, 24% had pain, and 8% were asymptomatic

Did a large or small volume of contrast material extravasate? Did you have obvious tissue damage? Usually with small volumes of extravasation no significant damage or long-term sequelae occur. I'm not sure whether your observations of your veins indicates there was a phlebitis that occurred. I think you would have been told at the time. Has a physician ever examined your veins and advised you?

This requires in person exam by physician to diagnose what happened recommend see if physician primary care ER or go to the imaging center where you had the CAT scan