tenants old utility bill tacked onto mine

My current tenant and I signed a new agreement 2 years ago, so she is still at the same address. This 2 year old agreement has the heat and hot water, both natural gas, included as part of our monthly rental agreement.

Tonight while talking to the gas company, National Grid, I was informed that HER past due bill was tacked onto my bill. We are talking $3,000 and this is at the same account service address. Electric is still in her name.

While I am not surprised that she owes this money to the gas company, how is this my problem if I am under rental agreement with her heat included and she is CURRENT with rent?

Am I right to want to laugh at this? My account with them for this address started 2 years ago, and I am 100% current. Go after her, that is her problem. Am I wrong?