Combined feeding and sickness

[glasgowmum13]

I have started introducing a bottle of formula in evening
Partly for a good nights sleep(it's working!!) and also to get him used to it for when I go back to work.
He is 8 weeks and exclusively BF up to starting this a week ago.
Problem is he is sick the next day when back on breast!
HV is stumped and he is gaining weight well.
Any ideas? On aptamil and just tried sma last night - but totally coated in sick this morning so think that's definately worse!!
Help!!!