Hi there...
Folks who have
Diabetes have multiple reasons for red rash on the neck.
However if its present for 20 days, appears to be probably some sort of Photoallergic reaction & can possibly be related to your medications also.
A more detailed history of your medical issues and the treatment you are on will be useful as also some clear pics of the neck area to make a pucca diagnosis and it's not wise to be guessing given your DM status.
Do connect ONLINE using this App's INSTANT ONLINE Consultation or related app's like Apollo 24/7 🙏
Will ensure its cleared ASAP.
Health Tips
Avoid OTC products, home remedies self medication and Dr Google's suggestions 🙏