help! dog keeps vomiting

please help! (i am an old member from about 4 years ago!)... i have a 2 year old silver lab who weighs about 120, but according to the vet, is not overweight, he definately fits his build...he is HUGE. he is as tall as an english mastiff. anyway, he has been vomiting off and on for about 12 hours. 3 times in 12 hours. it first started out with him vomiting up his food. (no change in food, nothing he got into, no change in environment) now it's that nasty mustard color. since he is a big dog, i worry that his stomach is turning. it's almost midnight on friday night... he is however allergic to corn so i wonder if maybe he could have season allergies as well since if you have an allergy, they can be prone to other allergies. any ideas???