Double - even triple check your lease to be sure which utility payments are, and which utility payments are not contemplated in the lease. If you're sure it is not, write your landlord and tell him or her so. Be prepared to pay your rent into an escrow account until the matter is settled. If you landlord doesn't back down, consider consulting with an attorney and asking the attorney to take a look at your lease and offer you his or her legal guidance.
Disclaimer: This does not constitute legal advice. No attorney-client relationship has been formed through this answer.