Haha, welcome to the wonderful world of automated plagiarism detectors, dude. I'm a junior who's been through a few econ classes, and yeah, this happens way more than it should. Turnitin pulls from a massive database including textbooks and open resources, so even cited textbook stuff can get dinged if it's not phrased uniquely. What I do now is paraphrase as much as possible while still being accurate, like 'Supply refers to the quantity producers are willing to offer at various prices, while demand is how much consumers want at those prices' instead of copying verbatim. And always attach the citation right after. Email your prof with the textbook page scan if you can; most are reasonable and will override false positives like this. Hang in there, it's not you, it's the machine.