Concerned about cyst-like bump on penis. noticed it about 6-8 weeks ago. it's located at the top of the shaft on the underside. when flaccid, it's rather firm, very small, no pain. but it gets rather large and stings/burns when erect. what is this?

Need examination: I would recommend you be examined by your doctor. It is difficult to diagnose on this platform as we cannot see your bump. There are many possible causes such as a cyst, which could be infected or not, a genital wart, a blocked gland in the skin, or others. Your doctor should be able to easily diagnose this with an examination.