Vein in my right arm randomly feels like it's cramping.

Muscle cramping? : It's unlikely that a vein is cramping, but more likely that the underlying muscle is cramping. This might be from muscle fatigue after a lot of activity and use of that particular arm or hand. Local warm compresses may help. See a clinician if you also have numbness, lightning-like sensations, swelling and/or redness, or weakness in that arm or hand. Hope that helps.