If it is normally higher than that, which it should be, was there another event that was occurring at the same time? Like playing your audio system really loud?
Did it do this just once and then never again?
If your car and anything in it is drawing more current than the alternator can supply, chances are than you will get dimming lights and a drop in battery voltage. A sure fire way to prevent this is to get a bigger alternator or have the stock one re-wound to a higher amperage.
High output aftermarket alternators are availible from AAMP of America and Ohio Generator.