Sources tell TMZ ... a 911 call was made from the CBS lot in L.A. -- where the "Skating" set is located -- at 11:43 AM PT.
We're told Smith -- who stars in "Sonny with a Chance" with Demi Lovato -- was throwing up and having difficulty breathing ... and had to be taken by ambulance to a local hospital.
Sources connected with the show tell TMZ Brandon was sick when he arrived on set today -- and it's unclear if he'll be able to skate during tonight's show.
UPDATE 1:30 PM: A rep for the show tells TMZ, "He was throwing up on set and we think it is food poisoning."
UPDATE 3:40 PM: We're told Brandon will not be performing on tonight's show.