My first thought about the Thai food/smoked mackerel is that it could be the fish sauce in many Thai dishes. I think the fish sauce is fermented. The other reason - I have just found out via a Google search - could be peanuts.
I don't have nasal polyps, but I do have a chronic condition of swollen nasal mucous membranes. One negative - among many - from this is that I snore very loudly and sometimes wake up breathless. I kind of take it for granted that I will always have nasal congestion. I try not to take too many anti-histamines, but sometimes I have to do that to relieve it.
So, I can relate to your condition, most definitely.