Not baby related but if you know anything about dogs please read

I have a 4 year old French bull dog and today he started throwing up a bright yellow liquid he was on the stairs when he did it and then all of a sudden his legs went completely stiff and he fell down the stairs. He laid on the ground for a few seconds almost doing the splits and then he slowly got up and was fine as if nothing had happened. Does anyone know what this could have been? I’ve been keeping a close eye on him since but nothing else has happened. It scared me, I honestly thought he was dying