So, usually a rash is not anything serious so do not panic. How's your hygiene? Sometimes we can get balanitis which is usually due to not cleaning properly and leads to inflammation and rash on the penis. It could be fungal as well like jock itch. Those are two pretty common things. https://www.healthline.com/health/rash-on-genitals#causes A doctor can usually visually inspect it and give you an answer or swab it. However, I'm sure it's not the greatest time to go to the doctor with all going on around the world health wise. One thing that often helps is over the counter cream, hydrocorisone. Unless you can trace this as being an allergic reaction, I'd try that first. You can get it at any pharmacy or store type of place that sells otc products.
Balanitis is a rash on the head of your penis.
When you were masturbating, did you use any creams or lotions, or something else for a lubricant? Could you have had an allergic reaction to that?
The red line - is it warm to the touch? Is it painful? If it's either of those things, please tell your guardian or another trusted adult. It could be some kind of infection that you'll need to see a doctor for.