Why would my breastmilk not be good for my newborn?

I just had my baby boy on june 1st. I've been breastfeeding since he was born and everything was fine up until last wk. After every feeding he started being fussy and would throw up so much. Not spit up but throw up and it would shoot out like a water fountain. I figured it would be something I'm eating or maybe he has acid reflux. But I gave him formula for 2 days and he did not throw up like that. So its not acid reflux. All that I eat is cereal, milk, oatmeal, a lot of fruit, bread and cheese. At first I thought maybe he was lactose intolerant but like I said he did not throw up with the formula so that could not be. Then I started thinking it was the fruit or the pineapple juice I was drinking. But I stopped drinking and eating those and he still continues to throw up. I'm so bummed I want to breastfeed so bad. What do you think it can be or what should I do? I took him to the dr and he said it was probably something I ate but what? :(