It is not uncommon for a La Nina to follow an El Nino.
SCRIPPS issues forecasts. Joe Bastardi points out they are not always right, but if they are right this time, he says:
Its forecast for this event, if real, would be spectacular. Not only would it be the biggest El Niño to La Niñas transition, but the strongest La Niña on record.
NOAA is predicting La Nina here.
The global effects of La Nina: