It could mean one of two things. The first one could be that they think that you don't have an appealing body and therefore it's "hilarious". If that's true, then that was quite rude of them and you should just disregard them.
The second reason is they are immature girls who still get embarrassed for whatever reason when they see a boy who is shirtless. My sister is currently at that stage and most middle school girls are that way too. And if this is the case, you should also disregard them because they'll grow out of it soon.
So I wouldn't worry about it if I were you. Most girls will always be weird and filled with giggles and there's nothing you can do about it. That's why just go with it. Personally I'm not really like that but my friends are. All I do is just ignore them, roll my eyes, and/or ignore it. It's quite simple.