A retail coworker snitched about me fake calling in sick

So I joined this retail part-time of selling clothes a month ago, and after two weeks on the job, I got a call back from my internship manager they wanted me back part time for the rest of the year so I put in my two weeks but for my last shift I was going to call in sick and I told a coworker who had been working there for a few more weeks than me that I would not be coming in for my last shift as I do have an exam the next day as I am doing summer school, I thought she would empathize with me being my age and a student and I trusted her enough to not snitch, I called the store this morning to call in sick and was told by the manager on duty that the coworker I confided in had told her about my made up sick story.
I still wonder why you would tell this to a manager, even though you said you hated the management in front of me. I could give two less damns about the manager yelling at me on the phone just felt betrayed as my trust was broken.