What does an allergic reaction look like?

Has anyone's little one had an allergic reaction to foods yet? Other than dairy. My baby is currently sick with an upper respiratory virus but he's in good spirits. He's had banana mixed with other foods but the last two days he's had banana alone and has pooped upwards of 6-8 times per day and is developing diaper rash AND is super fussy when he poops. All not normal. I was thinking it's just because he's sick and pushing out the virus but now I'm wondering if it's the bananas. Poor guy is pooping constantly!