Any idea why my damson jam has a disgusting bitter aftertaste??

[Carmenere]

I'm stumped. I made damson jelly yesterday and it worked really well inso far as it set beautifully, yet it has a revolting bitter/metallic aftertase which renders it useless. Actually I had a bit on my toast and now feel a bit sick!
I diddn't use an aluminium pan btw. any ideas?

leaving stones in can make jam bitter? although really hard to stone damsons I know

ah jelly

unlikely to have the stones in then

Oh, I did strain them out at the end and any recipes I looked up said it was ok to leave them in.

it can make it bitter leaving them in too cook