My cat Lucius (oh how I miss him...He isn't dead btw, he lives with my mum) did this all the time. He was OBSESSED with cleaning me.
He would pin my arm down with his paws and go to town. For a moggy he's a tall well built cat with a lot of muscle behind him, he's a bugger to get off once he's started.
Oh I miss my baby, he was my first cat (not just a family cat) and I absolutely doted on him
