My rabbit keeps shivering!!?

My rabbit is usually a pretty active little guy, well sorta. When I enter the room he usually greets me at the front of his cage. Well today when I got home from work he wasn't there to greet me. He sat in his hideout, which I thought nothing of, maybe he was just tired. Anyways I checked on him a couple hours later and he was moved to a new spot by his food dish where he was slightly shivering, enough to catch my eye anyways. So I picked him up and held him on my lap on the couch, which he seemed to enjoy. I am curious, is this something serious. It's a slight shiver, it's not constant, I'd say every 20 seconds or so. He doesnt seem to be grinding his teeth at all, although he is purring slightly. Please, anyone who has experienced this, let me know what's going on with my little French Lop