My 11 week old spit up.....?

something clear that almost had the same consistency of drool but it was a pretty good amount, was projectile & he choked on it. He was laying on his back & started fussing like he does before he normally spits up (the white spit up) but this time it was clear. Does anybody know what this could have been? Is it normal? Should I be concerned? He's not acting sick or anything. He's also spit up (normal kind) more often than usual today, but not an extreme amount.