Glitch with Canon 300D?

I've been taking a few 'bulb' pictures recently, and on one batch noticed a
slight lightened colour cast down the right hand side 10% of the picture,
similar to how you might with film where a bit of light has leaked in. I
originally put it down to some light entering the lens from a source I
hadn't seen in the dark, but took some more today and got the same thing.
Any ideas what this could be? I've not noticed any leakage of light on
settings other than bulb before. Is my camera/lens bollixed? Any suggestions
gratefully received.