Terrible tummy upset on my 3rd day of taking huel - whats the reason for it?

Sorry to hear that your experience wasn’t what you expected. I am no expert on the matter, but some people have reported some changes in their bowel habits on Huel, but not to the extend you did: Errrm. Huel Poop.
Maybe you find some answers there - otherwise there will certainly be someone better qualified then me answering to your post soon.