You can use this expression when:

• There's a bad situation.
• It seems like the situation is going to get better, BUT...
• Something else bad happens.

For example,

Just as I started to think that I was going to get a few minutes to relax, the baby woke up and started crying at the top of his lungs.

You can always use it in the opposite situation, when it seems like something bad is going to happen, but then you get saved:

Just as I started to think that we were lost, Samantha found the trail again.