(Turns out I posted this as a new thread. I can't Internet.
It won't be possible for us to tell you if your suspicions are correct or unfounded without seeing a detailed breakdown of what your bill has for charges.
Virtually everyone goes through "sticker shock" on the first bill. You will always, always pay in advance for any service which can be billed in advance (so, everything but water, natural gas, and electricity). Because all billing companies run on bill cycles, this means your first bill will always be higher than your normal monthly bills. Gotta get that dough up front, you see.
My Frontier bills have been straightforward and they detail where every penny is going. All is likely well, especially since most Frontier quoted rates are "promotional" rates that are adjusted through bill credits.