Acidic, sour or bitter?
tis always tricky determining what someone else is tasting via a webpage.
Short answer in rough order is:
...all of it really!
To determine which is causing the problem you need to change a lot of variables one at a time and there is no "one size fits all" answer.
A good level of acidity is rarely bad and in most milk based coffees some acidity is mostly a good thing but sour is rarely good.
Confused?* Think Coke Cola for acidic, you can feel the acid on your teeth but it doesnt taste sour like citrus (even with coke levels of sugar) and for bitter think panadol dissolved in your mouth.* Grapefruit would be bitter/sour.