When I play with my little one like bouncing her on my knee I hear her belly with liquid swishing around, a sloshing sound. It's really weird. Is this normal? 

x

I always heard this when my nieces belly's were full of milk. I'd always stop in case they puked all over me LOL

Normal from what you has described, it'll be sloshing not long after their milk feed which hasn't digested yet.