I mixed up a salad in a glass container I added dressing and goat cheese crumbles and shook it up. The next day when I went to eat it all the cheese ended up in a big congealed lump in the bottom of the container. Did I do something wrong?
I imagine the cheese just "melted" in the presence of all the liquid in the vegetable and dressing.
You should try not prepare you salad a day before; or at least do not shake all the ingredients until you are ready to eat so that the more fragile ingredients do not wilter in the contact of the dressing, or don't add the dressing at that time, have a small container that you can use for the dressing.
If you are short on time, just have all the separate ingredients in different containers and assemble in the morning (or as close as possible to when you will eat it).
For example, have a look at this: