Was what I felt—fear and trembling—after sensing something pass behind me while reciting the Qur’an after Fajr prayer a jinn?

You should not pay attention to what you felt, for a believer is not harmed by such things. If it was a jinn, Allah will protect you from its evil by seeking refuge in Him, holding fast to the rope of Allah, reciting supplications (dhikr), reciting the Qur’an, and relying on Him. If these were mere imaginings (which is more likely), then cast aside these whisperings and rely on Allah.