I am looking for an idiom that could be said under such a situation:
You say something to someone and then you hear all about that someone's problems.
Thanks a lot
What about 'open Pandora's box' idiom. Could that be used? I know what it means, I'm just thinking whether it is not too strong and it could be used in this situation.
Well open Pandora's box would not be specific for problems, it refers to unnecessary and irrelevant talk in which you are not interested.
If you want to know what someone is thinking, you could use 'a penny for your thoughts', but it is not used much for serious things. If you didn't intend to hear about them, then I would go with Buggles' open the floodgates.