Take a look at the sentence again:
The problem occurs between hypochondriac and when. Julie is a real hypochondriac is a main clause, so it can stand alone as a complete sentence. When her stomach hurts, she is certain ... begins the next complete sentence. Because these two parts have only blank space between them, they make a fused sentence, a major sentence error.
To fix the problem, you could put a period after hypochondriac and then capitalize the W in when.