Hi. You don't mention the type of cocoa powder. The "Dutch process" cocoa is not really that bitter, having been treated with alkali (sodium bicarbonate I think, in this case).
If the cocoa powder that you took was terribly bitter, that is socalled "natural" cocoa, which was not treated with alkali. I should think that would be more likely to irritate the flukes.
So, can you tell us what kind of cocoa powder you took? Tks!