When it's Raining, It's Pouring is an idiom that means the rain is coming down heavily, but in a figurative sense, it can also mean a situation is going from bad to worse.
Example: "When it's raining, it's pouring, let's get under the umbrella!"
Another example: "My relationship is going from good to bad, when it's raining, it's pouring!"
Read more about the idiomatic uses of When it's Raining, It's Pouring in Pop Culture