very strange bump on my penis; only can find during erection

I have a strange situation that I haven't read about yet so far on these discussion boards that I need an answer to. I have a very small cyst-like lump on my penis, but it's difficult to find for one important reason- I can only feel it when I have an erection. It's on the shaft of my penis about half-way up, it's on the side, it's not movable, it's under the skin, and you can't see it through the skin. I've read about sebaceous cysts, but they say that these can be seen and they are commonly found on the scrotum. I don't feel any discomfort at all. I constantly try to feel for the small lump when I am flacid, but I can't find it at all. It's just not there, or at least I can't feel it. I'm 33 years old, have a healthy sex life, no pain or discomfort anywhere. If you have any suggestions or advice, please let me know. I would greatly appreciate it. Thanks.