If you need a sick day inform your employer you are unable to perform your duties. Leave it at that. They do not deserve a reason or a doctors note. They cannot require you to get one for a day off work sick. I fucking hate how most employers think they have the right to decide if you're sick enough to warrant a day off. You're not a slave, or a drone (well to them you are, I'm saying in reality you aren't)
Sick days can be anything. Even mental health days are fine. Your employer does not need to be given details. They aren't your parents or a doctor. They aren't your therapist.
Just call in sick and turn your phone off. If they try anything like penalties or writing you up you can easily fight it.