Son eats so little?

I have no idea where to post this but after my son received his 2 month shots he's been eating 2oz a feeding (when he'd eat 4oz-5oz). It's a looong day of him being hungry, fussy and refusing to eat. He refuses to eat even when he's hungry. I'm so worried. Max he gets 20oz-25oz and he's 3 months. Shouldn't he be eating more? He did have a horrible reaction to the oral vaccine, so I'm not sure if it's due to that. He's formula fed. Any ideas or experiences?