If I was on Prozac (fluoxetine) comfortably for 6 years and then developed vertigo could it have been a new side effect of the drug even though it took 6 yrs?

Unlikely. Not unless you added another medication which may interact with prozac (fluoxetine). Some medications do have side effects that can develop after several yrs, but Prozac (fluoxetine) is not one of them and vertigo is not one of those symptoms as far as i know.
Doubt it. I doubt it. However if there was a change to medications or supplements, diet, etc- that impacted Prozac (fluoxetine) metabolism that could be an issue. Best to see your physician or your ENT specialist to determine cause.