On the way to work this morning (I've just arrived), the rear end on my 'S' started to feel... odd. It felt like it was drifting around corners and the whole bike felt very unstable, even in a straight line. I was sure that I had an imminent flat, so I pulled over to have a look. No flat and no obvious cause of the problem - I visually checked wheel alignment, grabbed the wheel to check for blown bearings etc., but I can't see anything. I've got to ride the bike home tonight and frankly, I'm scared. Can anyone suggest a cause (shock problem...?), any possible remedial action and what I should do after I've ridden home at 20mph?