Hi, about two weeks ago, I noticed a very small (just the size of a dot from a ball point pen) but hard zit on my penis shaft just after sex. Few nights ago I masturbated and slept. Next day when I woke up, the zit grown into a lymph like, yellowish, round thing on my penis shaft. It hurts a bit if I press. It moves with the skin and feels nothing under the skin. Could it be lymphocele or an STD? Should I be worried?