It LOOKS LIKE- you had small bubbles close to the surface. When you sanded, you opened them up. What you have left is a void that is filled with sanding dust, which makes it bright white. You can check it by blowing it with some compressed air, either from a compressor of like a can of computer duster. The solution is a little bit trickier... you will have trouble breaking the surface tension enough to get epoxy to fill into those spots. it's a lot of work, but it looks like you've sanded into the weave and you'll need to add some more epoxy anyway.

IF IT WERE ME- (this has happened to me before) I would do all of my sanding now, because I'm guessing you'll probably open up more bubbles. Blow it all off with compressed air. Then heat up the resin before you mix it, do small batches, and wipe it on with a lint-free cloth, or brush it on lightly and use a high-quality 3M plastic spreader and hope that it fills the voids. Then sand it with 220. If you've still got those voids after sanding, I don't thin you'll have any luck with more coats. My solution- which was at the suggestion of the coating manufacturer- was to put on an extra coat or two and sand down. pretty aggressively until the holes are filled, then put on your final couple of coats. The finish is probably thin enough to fill the void, where the epoxy isn't. The problem is the finish isn't as thick as the epoxy, so it takes a coupe of coats.

Since you're working on a car you may be spraying- so it may be relevant that I sprayed Awlcraft 2000.