What Could Cause A Persistent Bad Taste And Dark Green Coating On My Tongue And A History Of Chronic Sinusitis?

I have been experiencing a bad taste in my mouth over the last few weeks. i go to the dentist regularly and had my teeth checked and cleaned about two months ago and my teeth are good . I noticed today upon scraping my tongue, there is a very dark green coating towards the back of my tongue. I scraped it twice about 2 hrs apart and it was there twice. There weren t any foods or drinks which correlated. (all I had was a BLT at about 2 hrs prior to the finding, and after brushing my teeth) any idea what this could be? I do have chronic sinusitis? Im thinking maybe sinus infection?