Sorry, but I will go against the majority advice and say that it MAY be the ejuice.
I've received flavorings that literally "went bad" in 10 days. And I just bought it! (these are subtleties you figure out once you start DIYing for yourself).
You don't know how long a super large bottle of flavoring has been opened, that is being mixed into ejuice, even if the ejuice itself is freshly made. It also depends on how many times the flavoring and other stuff has been opened and closed (exposed to air and light).
Your comment "took a few hits without hitting the fire button and it tasted fine" leads me to believe that when heated, SOMETHING in the eliquid is not taking to heat very well.
Sometimes that is because of too much sweetner added to the ejuice so when heated it tastes awful because it is actually "burning" even without high heat. sometimes it's because there's too much VG and your coils aren't handling very well once button is pressed. Other times, it's just old flavoring. Or even old nicotine. And yeah, sometimes it's your coil or actual device, but not always.
Just wanna put this out there.
Have yu tested with different eliquids?
Yeah, what she said (Racehorse)........... very logical advice.