Explicit details in the text that reveal more about Jenna's feelings include her actions and expressions: she "rolled her swim towel into a ball and shoved it quickly back into her bag," which indicates a sense of urgency or frustration. Additionally, "water welled up in her eyes, making her vision blurry," suggests she is on the verge of tears, indicating sadness or disappointment. Her grabbing the bronze medal and her thought, "Time to go home," imply she may not feel proud of her performance despite having won a medal.
Based on these details, the most reasonable inference about why Jenna is upset is: She did not swim as well as she expected.