Many reasons.
From upset stomach to dental/gum disease. Check with dentist.
Keep nose clean-dripping nose may be the issue.
Acidity (GERD, Hiatal Hernia, H-Pylori infection)
, bad tongue (brush your tongue),
take multivitamins (B complex),
check liver function (bile tastes bitter).
Keep yourself hydrated.
Clean or change air filter in your place.
If you are using canned spray to clean your computer…it has bitter taste added to the nitrogen.
Difficult to say what exactly is the reason unless you tell more.