Why Do Managers Do This (USA)

So, I am a crew member at my McDonald's. Im currently looking for a different job, have been since I joined McDonalds since this job is just to pay bills until I can get something better. Well, I called in today because I feel sick to my stomach and have been getting nightmares about work to the point that I feel very ill (I also walked in the rain to work yesterday, so I think this is added to that as well). It's currently almost 6am as I'm typing this, I work at 8am but my ride to work needs to be at his location at 6:30 so he drops me off two hours early. My manager, who is extremely nice and very sweet, but is still my manager, says this to me after I call in the allotted time if you're going to call in sick: "Get someone to cover your shift and you might want a doctor's note to be on the safe side." I'm doing neither because it isn't my job to find someone to take over for me when I know we're going to have a full house of employees. And being in the US i can't afford a doctor's appointment even with my insurance. But why do managers do this and play the power card? I kind of get the doctor's note if it's a continuous problem but I do not call off that often. I'm just confused.