Under the lease with the landlord, you and your roommate are jointly responsible for the utilities. When you left, your roommate ending up paying for all them. She, as a third party beneficiary, could sue you under the lease for your contribution to the lease--e.g. since you agreed to pay utilities to the landlord under the lease, and your doing so benefited your roommate, she may have grounds for a legal action to recover those costs from you. (She could also recover your share of the rent from you.) If you had an agreement when you left that would absolve you from that responsibility, that would be one thing; but in the absence of that, when one tenant who is on a lease breaches by moving out early, he or she can still be held responsible under the lease by either the landlord or the other tenant.