Mine occurred when I went back to playing bass and after a prolonged session my little finger had a pronounced bruise on the finger tip (that's b-r-u-i-s-e, pronounced brooz :), sorry couldn't resist an old Spike Milligan throwaway line).

My left hand fingertips are generally pretty solid and calloused from acoustic guitar playing but the fingers probably fret the bass strings in a different place and that my have caused it.

As an aside I now have two sets of callouses on my left hand fingers and a separate set on my right hand first and second fingers as the size and width of the bass strings sure knock the hell out of your fingers early on. But as a bonus I can fingerpick all day and night on my acoustic with this new protective layer!