Take a look at the sentence again:
For a sentence to be a comma splice, you must have two complete sentences joined with a comma. In the example above, there is no comma. Julie is a real hypochondriac is one sentence. When her stomach hurts, she is certain ... begins the second sentence. Because there is only empty space between the two, you should call this error a fused sentence.
Remember that both comma splices and fused sentences are major errors. They make your reader think that you cannot write a correct sentence. Because the sentence is the most basic building block of a piece of writing, comma splices and fused sentences make you look like an amateur!
To fix the problem above, you could put a period after hypochondriac and begin when with a capital W.