Sounds like the nurse or tech hit a valve in the vein and/or the IV contrast irritated the vein. Since you have had this for a month, I would recommend having the site assessed by your PCP/Family Physician.
It sounds like a bit of blood under the skin, which will eventually disappear. Probably caused by perforating the vein during the insertion of the trochar. It happens.
If it seems to be red, or becomes painful, head for the doctor. That would suggest infection.
Otherwise, a cool compress.
Hello~The nurse or doctor may have hit a vein or was too rough, this can sometimes cause a lump or slow healing. Try putting some ice on the site.