My disposal has always worked just fine, but today it's sending water back up into the sink after it's gone down once the switch is turned off? A clog? Does this necessarily mean a plumbing call?

When you disposal is running it expels water under pressure. If there is a blockage some of the pressure is obsorbed by water backing up ealsewher including up the vent. Whne you shut it off gravity brings everything back to level. Yes you have a drain problem. Call a plumber, yes if you can't rod your pipe and possibly remove the trap below the sink. Do not put drain cleaner into your disposal