Final answer:
The speaker's behavior changes include increased self-reflection and decreased academic obsession.
Step-by-step explanation:
The changes described in the speaker's behavior as the stack of notes under their mattress grows thicker are increased self-reflection and decreased academic obsession. These changes can be inferred from the speaker's statement about tucking their college notebook under their arm and going off to listen to a lecture. It suggests that they are becoming more focused on their own thoughts and less preoccupied with their academic pursuits.