Vomitings after having formula

Hi there... I tried giving formula to my (7 week old) baby two days back, as I wanted him to bottle feed atleast once a day.. As he dint seem to like the formula when we did it last week and the week before, this time we went with 75% breast milk and 25% formula. He dint seem to like it and then had 5 big vomitings through the evening. He reacted the same way earlier too when we tried giving him a bottle of 100% formula. Any idea why this happened? Has anyone else been in a similar situation? I feel so terrible having done that to my boy.. Any insights would be really helpful.