Chances are that the rental agreement you and your gf signed recited that liability under the contract is joint and several. That means the apartment landlord can sue you, or sue your girlfriend to recover money they think they are owed for your breach. Or they could sue both of you. Generally a creditor is going to sue whoever they think cares more about their credit. It may be that there is no lawsuit, but your gf and the apartment have worked out a payment plan.
But if you and your gf jointly took on this obligation, and you both left, and she is making good on the payments, it is perfectly appropriate for her to sue you to recover your share. As between you and her, unless there is some writing to the contrary, you and she agreed to share the obligation.
IF she takes you to court the court is likely to enter a judgment for the full value of the obligationand courts don't make payment plans. $25 a month is not very much. So on these facts, yes, she could sue you and have a decent chance of recovering from you.
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