My penis smells rancid.

Hi guys, I have a question about my penis. I am rather embarrassed so I've not consulted a GP or anyone else. I've looked online but I can't find a definitive answer.
When I pull my foreskin back there is a very rancid, fishy smell. It smells like stale urine almost. It doesn't always smell, sometimes it smells a little bit and sometime it smells very bad. It is seriously affecting my sex life with my girlfriend, as she refuses to give me oral sex and sometime if she gets a smell off the penis whilst having foreplay it'll put her off sex.

I've noticed that sometimes during foreplay when I pre-cum the smell goes away, as if it was trapped under the layer of liquid.

I am 20 years old, I was once a day and I have good hygiene. I've had this smell for about a year - a year and a half now.

Any ideas on how I could get rid of the smell? I'm seriously desperate!

Regards

A.L