What’s wrong with my 16 week old?

Firstly I’ll just mention that she had her vaccines 2 days ago and where we are it’s very very hot at the moment. So recently she’s started being really fussy with her bottle as she’s get distracted very easily. Then she started eating less than before like usually she would have five or six bottles of about 7oz and then she started having lots of bottles of maybe 5oz. And this morning she has about 2oz and then refuses to eat more. Even though she’s eating much less she’s still spitting up. It’s also been taking her a lot longer to burp these days. I dread feeding her now because it’s so stressful and difficult. Anyone know what could be happening?