Random bump on vein. what could that mean?

Soft veins: As you know, veins are very soft, so having a bump is unusual. Unless the area is red and tender, what you are feeling is probably something in the same area that is causing the bump such as a cyst on a tendon. Depending on the area you are feeling this. There are lots of structures that run under, next to or as part of the vein as it travels. Observe this unless it has other symptoms that worsen.

Hard vein.: Veins are usually soft and compressible. If your vein has a hard area then this could be a localized area of clot called superficial phlebitis. It also may not be the vein at all but something on top of the vein. See a vein specialist.