The bitter taste could be bile coming up. This is normally caused by acid reflux...but you aren't complaining about stomach issues...so hard to say.
Many illnesses and injuries can cause taste and smell disorders, including colds and head injuries. Some drugs can also affect taste and smell. Most people lose some ability to taste and smell as they get older. Treatment varies, depending on the problem and its cause.
My ex husband once broke his nose and couldn't smell anything for years. Then one day, ( just before baby #3 was born ) he re-broke it...and he was smelling the oddest things...not to mention the pleasure of dirty diapers. lol
People with IBS, Crohn's, Epilepsy, etc seem to smell things that aren't there as well.
I have IBS and GERD...I smell weird things all the time. Every time I go outside the fresh air smells like melted crayons to me.
Odd smells can be triggered by temporal lobe seizures, epilepsy, or head trauma. Phantosmia is also associated with Alzheimer's and occasionally with the onset of a migraine.
Common olfactory hallucinations include lots of odors. Sufferers report smelling hydrogen sulfide (rotten eggs), bad perfume, garbage, a gas leak, wet dog, pungent body odor or spoiled fish or feces.
Smell disorders aren't that rare. According to a 1994 survey, 2.7 million Americans have some type of olfactory problem, including anosmia (the inability to smell); hyposmia (a decreased ability to smell); parosmia (a distorted perception, instead of flowers, you smell rotten meat), and phantosmia. Another 1.1 million people have issues with taste (smell and taste are inextricably linked) including ageusia (the inability to taste); hypogeusia (a decreased ability to taste) and dysgeusia (a distorted ability to taste).
Phantom fragrances can be produced by one or both nostrils and can waft in and out of a person's life over the course of a few hours or a few days or a few weeks.
Anyway...there are many reasons why we "smell things"...but I don't think you have any of the "bad reasons" as you haven't mentioned any other symptoms other than the bitter taste...which leads me to believe it's acid reflux, or some other stomach condition.
Best bet is to see your doctor, just to be sure.
well just take a old fabreeze can and add: