I have the same thing and have seen it on at least one other person lately ie)my trainer who is jacked. Ours both protrude with the slightest amount of weight on that leg. I was told probably about 10 years ago or so that its a vericose vein. I also have similar bulges in my arms which I personally believe to be the valves in my viens.
None of mine hurt, not even to touch, but Ive had the one on my outer calf for over 13yrs and have never had a problem with it. Only thing I know for sure is that the leaner I get the more pronounced the lump becomes.
Hope that gives you some peace of mind.