Take a look at the original sentence again:
Sylvia shivered in her silk shirt, the difference
in temperature between the hot road and the cold library was extreme.
You wanted to fix it this way:
Sylvia shivered in her silk shirt, because the
difference in temperature between the hot road and the cold library was
extreme.
Because, a subordinate conjunction, would fix the problem in the original sentence if there wasn't a comma coming before it. When a subordinate clause follows a main clause, you typically don't use any punctuation to connect the two.
You might want to consult the rules for fixing comma splices and fused sentences.
Go back to the sentence to try again.