To be perfectly clear.  Many engine related problems seem as if they happen over night, but in reality it takes many weeks or months of something not working properly for the problem to finally show up.

With that said, I would love to hear the explanation that your mechanic has provided as to what he means by "compression problem".  I dare say that $7,000 is a ridiculous amount of money to pay for to have a stock engine rebuilt on any mass produced automobile.  Myself, I'd probably get a second opinion.