What causes something that looks like a ring-shaped swelling on the penis after prolonged masturbation?

Hello. First of all, yesterday I masturbated without any lubricant; one session started at 10 AM and lasted for 1 hour, and the other started at 12 PM and ended at 2 PM.

A short time later, I noticed something that looks like a ring-like swelling in the middle of my penis. **I don't actually know if it is a real swelling or not, by the way, it just looks like one.** It is only on the front side; there is nothing on the back. It has gone down a bit but is still there. I think it might be from skin irritation, but this is the first time I've experienced something like this.

I don't have any pain, soreness, or difficulty urinating.

I would be very happy if you could give me some advice or an idea of what this might be.

To describe the scale: I am using the side rim height of a standard European water bottle cap as a reference for how much the ring bulges/expands from the skin. When I first noticed it, the width/height of the band was equal to the side rim of a bottle cap. Right now, it has reduced to about 75% (0.75) of a cap's side rim height.