The Chao Phraya river floods every year at around this time because of monsoon run-off in the north.
Central Bangkok - the parts where most tourists visit - are protected by levee embankments. Last year was especially bad and parts of greater Bangkok which normally don't flood were affected.
Will it happen again? Depends on the weather.... be prepared and keep an eye on the news.
Here's the latest report I"e seen:
Phil from the safety hub about 4 years ago