My budgie keeps squeaking and collapsing and I don't know what to do?

My budgie has collapsed three times today. He has collapsed before but it was last year and he recovered straight away. I have watched him, and he tends to squeak really loudly, as if panicking, then spins round, and collapses. The last time he collapsed he threw up, and he is now really quiet and still. He seems distressed as he keeps plucking out his feathers and we don't know what to do. He was fine this morning though and was tweeting along to the radio.