Not sure if my cut is infected?

I grazed my arm a few weeks ago I fell of my bike the cut isn't that big and I didn't think anytthing of it so I didn't clean it I just left it. The cut is now red around it & a little swollen and the scab is abit of a yellow colour. Is it infected? It also doesn't hurt when I touch it just hurts when I strech my arm out. Thanks for any answers.