any ideas ?
Are you using condoms during sex? If not you may have a yeast infection if you have had unprotected vaginal sex. Are you washing your penis with soap?
This is starting to look like some form of Balanitis - what you have can be described as Balanitis anyway as this in simple terms describes red irritation (and other things) on the head of the penis.
However, there are several actual Balanitis conditions such as Balanitis Xerotica Obliterans or Lichen Sclerosus for short. What you need to do is to rule out, or clear any sexual related issues first.
The other possibility is a Dermatological adverse reaction to something that is in regular contact with the head of your penis. You can develop an accumulative effect of different chemicals in regular contact with the head of the penis. This can be soaps, shower gels, Lubes, skin cream / lotions, condoms - and so on. The best thing to do is exclude these from contacting the head and foreskin of your penis. Wash with water only, dry immediately afterwards to ensure you are 100% dry. You also need to abstain from sex and masturbation for as long as you can .. put in place what I recommend - rule out or treat sex related infections and you should see improvement within 4 or 5 weeks …