Liking other girls instagram and facebook photos

Me 18F told my boyfriend 20M (we have been dating since March, 5 months) it made me feel like he was getting bored of me whenever he liked other girls photos, he said “you’re funny” as in acting like it doesn’t mean anything. Does it mean anything when he likes other girls photos? Is it just casual? I always worry incase he’s speaking to them behind my back.