I think that it is a sign of the cat being a bit off colour.
Normally I would five you a link to my favourite cat website - but either it is down or I am being more stupid than usual.
http://www.fabcats.org/
Cats are good at hiding the fact that they are unwell and it is good that you have picked up on a sign.
If it were me I would panic and drag the poor creature to the vet expecting a diagnosis of some fatal disease. The sensible answer would be to leave it a day or two, as long as puss is eating/drinking and filling the litter tray.
It could be a fight injury and if you suspect this take junior to the vet tomorrow.