It vs That , which is correct?

apologies if this was asked before, but I cannot seem to find an answer to my dilemma. When we say something happens, for example "You/I've started to feel sick this morning; that always happens when x" or should it be "it always happens?" I think "that" sounds more natural, as "that" refers to the start of feeling sick, at least I believe so, but that "it" is bothering me a bit. Thanks in advance!

Since you're currently in the midst of starting to feel sick, I'd definitely prefer "this" over "that".

"It" sounds weird because it normally replaces a noun, while you're trying to replace an independent clause. This/that are pronouns that can refer to more abstract concepts, and it can subsequently refer to this/that once you've established a referent.

I've started to feel sick this morning; this always happens when I stay up too late. For some reason it doesn't happen if I was drinking coffee, though.