In sentences there is no such thing as primary or secondary stress. Primary and secondary stress are word internal concepts. Depending on which system you use to analyze sentences there are three (or more) different types of prominent syllables that we can find in sentences:
A STRESS is a syllable which has rhythmic prominence. An ACCENT is a stressed syllable which involves a distinct pitch change. Accented syllables are therefore more prominent than other stressed syllables. The NUCLEUS, sometimes known as the TONIC syllable, is usually the last stressed syllable in the intonational phrase (read sound-sentence). Each intonational phrase has exactly one nucleus. It is musically distinct from the rest of the sentence and has its own tune, which starts on the nucleus and carries on till the end of the intonational phrase.
The Original Poster's Question
I'm in a hurry.
I'm not in a hurry.
As a crude rule of thumb, nouns, adjectives,verbs adverbs and wh- question words carry stress. So do negative words, such as not and negative contractions such as can't. Pronouns, auxiliary verbs, prepositions, articles and so forth do not usually take stress.
Under normal circumstances, the first sentence will only have one stress. This will be on the first syllable of hurry. As it is the only stress in the intonational phrase, it will also be the nucleus and determine the nuclear tone. Everything before the word hurry is likely to be said at a low level pitch. This section is what is known as the prehead.
In the second sentence, there will be two stresses, one on the negative word not and another on the first syllable of hurry. They will both involve changes in pitch and are therefore both accents. The pitch will be low and level for the pronunciation of I'm. It will then jump up for the negative word not. The nucleus will be on hurry. There is often a misconception that the nucleus will be the loudest or most prominent syllable in the sentence. However, very often the onset, that is to say the first accented syllable in the sentence, is the loudest and often the most prominent. It is perfectly possible that the word not will be more prominent here than the first syllable of hurry.