Swollen back lip on cat

In the past couple of days it looks like my cat's lip has gotten very swollen.  I tried to look it up, but the only results I can find are from a front lip, not the back.  Is this something serious, and should we go to a vet?  What could be causing this?

 

Welcome to TCS.

I would definitely have it looked at by a vet. Some times it's allergy related or could be a dental issue. Either way, only a vet will be able to tell you for sure.