i'm no doctor, but just from googling, from what i see, hernias, cysts, blood clots are normally painful- i would guess that it's an engorged vein or something from blood pumping harder after exercise
anyway, google has alot of very similar results you should check over https://www.google.com.au/search?q=lump ... r+exercise
some say it's a reaction to certain plants in your diet like sunflowers/chamomile
edit: some people on bodybuilding.com say they have similar conditions, caused by tears above the muscle, and under the skin that show up after running
and if you're worried about it after reading through, you should get it checked out, i'm not qualified.