What causes these small red bumps (sometime itchy) after running in very cold weather?

I’ve noticed that when ever winter hits and the temperature plummets I end up with smallish red bumps on my legs and bum after I come in from running. They are itchy sometimes but they go away after my skin heats up again.

What are they and why does this happen? (Beyond the obvious reason that the skin is mad that it’s so cold).