It does not matter what she told you the day before. What matters is what she told you the day of, when you are doing it.
I recommend you discuss this with a parent or guardian, because if your ex-girlfriend makes a formal allegation, you will need adult help. Do not say ANYTHING to the police without a parent/guardian present, and even then, get a lawyer before saying anything more than confirming your name and address.
I also suggest, if this blows over, looking into local OWL (Our Whole Lives) programs, as you are desperately in need of fact based human sexuality education, including consent.