help! opinions please!

My LO is 5 months old and this past week and a half he has starting eating only half of his bottles but still at the same time each day.. he's started spitting up and he never used to spit up. I'm so worried about him though. I took him to the doctor and all they did was weigh him, he gained weight since last doctors visit but they said he might a little virus bug or something but I'm still worried.