Apple Music charged me the first day I signed up for a free trial

I seriously doubt this.

You may check what you have actually been charged for here:

See your purchase history in the iTunes Store on a Mac or PC - Apple Support

Likely your credit card has been verified and authorized for a charge, but the charge was not actually made. In this case, you will see no purchase using the method in the article above.

Otherwise, you had enrolled for a free trial earlier.

See sberman's reply below. IF you signed up now (as in first month) and IF you got an actual charge of $9.99 on your CC, then you need to contact Apple. We are users like yourself. I signed up for the service, gave them my CC credentials, liked the service, and did not get charged until month 4.

How to have Apple Music free for 3 months?