we don't really want to speculate. Another poster has already volunteered that it might be the soap you're using. If the problem continues, see a physician.

Any suggestions referring to diagnosed medical conditions will be deleted,
The Love Goddess

right on the head of my penis swell up? It only happens in the shower.

I ment to say "right under the head of my penis".

You say this "Only happens in the shower" so I assume you also scratch the itch other ways at other times.   What is different?   You've already had the soap irritation suggestion which is a good one.  It might not just the soap as a chemical irritant...  inferior lubricant...  residue trapped under the foreskin or getting inside during the play.   It could be just physical irritation if different technique.  

If it bothers you don't do it...  let it rest...   find a different lubricant.
Best wishes,
skb