I recommend that you first check your company’s compliance policies because they probably require that any alleged violations of ethics, law, or Company policy be reported to HR immediately. That’s what your coworker was doing – making a compliance allegation, right? Your boss, therefore, should have followed the company’s prescribed process by reporting the emailed accusation of your ethical violation rather than handling it herself.
You’re probably best served now by connecting with HR. You can tell HR what happened and let them decide whether there is something to investigate. The benefit in having HR investigating the matter (other than the fact it was required) would be that they can review confidential information, substantiate or unsubstantiated the matter, and communicate with the individual who originally made these allegations against you. This includes conveying warnings against retaliation. Follow the process your company lays out in its compliance policies.
As a courtesy, you may want to give your boss a heads up that you are going to HR or have just consulted with HR so she is not surprised by a phone call from them.