The "Saved by the Bell" star's rep tells TMZ ... Dustin's struggling to come to terms with having the type of lung cancer he has -- stage 4 small cell carcinoma -- because it's normally caused by smoking, and he's never been a smoker.

So, we're told Dustin can't help but wonder if his cancer is the result of his frequent work travel over the past 20 years.

The rep says Dustin's worked all over the country on film sets and at comedy clubs for meet-and-greets ... and sometimes had to stay in pretty questionable hotels and motels, because there were no other options.

We're told Dustin believes some of these spots might have had mold or asbestos ... which he could have breathed in, causing his cancer.

As we reported ... Dustin just completed his first round of chemo at a Florida hospital, and will start physical therapy soon while he awaits the second round of treatment.

We're told his doctors have not determined what caused his cancer, but as we reported ... he does have a history of it in his family. His mother died of breast cancer.