Mimetic words like these are often quite unspecific and apply to a range of situations/feelings.
I would describe もやもや as basically the opposite of being relaxed and carefree. There's something about a situation (or your life in general) that worries you and you're not satisfied with it. You're kind of restless, feel uncertain about something, ... that kind of thing. It could also be a time where you just can't get yourself feel excited about anything and nothing feels like it'd be fun to do. You always keep on being worried or distressed.
So maybe that fits within the context of your manga? Sickness could make one feel down like this, and romance I guess can also evoke all sorts of emotions.
Alternative, maybe the people are speaking korean ;) https://twitter.com/idol_hangul2/status/1232879437575704577
Meaning of もやぁ as a sound effect
Hi! So I was reading a manga, and I came across a sound effect I never heard of before, もやぁ. It was used in the context of the heart squeezing/skipping a beat (not really sure). I have also heard of キュン but never this. Even searched about the word here but only got もや, which is like a hazy feeling of sorts. Not sure if the context is regarding sickness or has a romantic context. Any idea what it means?