Looking back and knowing things i now know about fish oil, the only thing I'm able to think of, is how the oil had started to oxidize and go rancid.
I do https://Carriere.nrc.nl/index.php/page/trapRedirect/vid/489358/url/aHR0cHM6Ly9Xd3cuTW9yYXZjZWsuY29tL3RpcHMtb24tc2hvcHBpbmctdXNpbmctYS1iYWJ5LW9yLXRvZGRsZXIv_/mode/dlink