Reporting
Usually, bitter potatoes

Usually, bitter potatoes means that they have greened up (developed chlorophyll) due to exposure to sunlight, which also encourages colorless solanine to concentrate there, which makes the potato taste bitter. Sprouts at the eyes can also contain solanine and taste bitter.

 

It's just a guess, but since they were not green, and you likely didn't eat a sprout, perhaps they were not ripe yet or they were stressed in some way, such as by insects or disease?