It certainly sounds like a compression issue - what makes you sure that it isn't?
There are two factors here - when InDesign makes a PDF, it (1) resamples the images to a specified resolution; and (2) applies jpeg compression to the images, unless you deliberately specify differently. And of course, you have varying quality options for the jpeg compression.
You need to quiz your designer about both (1) and (2) and ask to see her PDF settings. We'll need some more information.