What were the circumstances of Agnes' (Oscar's mother's) death?

Regarding the 1979 Oscar winning movie The Tin Drum, is there an explanation for Oscar's mother's death?

Agnes is officially married to Alfred, but she has an ongoing affair with her cousin Jan - Alfred probably knows that, but for some strange reason he doesn't mind (Oscar as narrator mentions that Alfred and Jan were really, really good friends... take it however you want). Oscar even notices at one stage, that "he has blue eyes like uncle Jan, not brown like mom and dad", which would suggest that Jan is Oscar's real father (although kids eye colour can change over time).

Now, we have few scenes that lead to Agnes death :

1. We can see that Agnes and Jan have sex.
2. Agnes, Alfred and Jan go for a walk on the beach and see a fisherman fishing eels using rotting cow's head - Agnes gets sick looking at it.
3. Agnes is very upset at the dinner with her husband (we later find out that she is pregnant, presumably with her cousin Jan). Her husband tries to convince her to eat eels - she initially refuses, then after an argue she eats them all.
4. Alfred is getting worried - Agnes eats only fish, which makes her sick. We can see Agnes eating fish straight from a can, then running to the toilet.
5. Agnes is dead - we can see her funeral, both Jan and Alfred are devastated.

So I could assume that Oscar's mother make herself sick purposefully in hope to either lose the unwanted child or to kill herself - if indeed the fish diet was the cause of her death (or was it something else?). But why would she do that? It seems that all three of them were quite happy in current arrangement and Alfred would probably accept Jan's child as his own (as he probably done with Oscar). Was she simply sick of living in such weird ménage à trois?