Would this be considered a case of plagiarism?
For coursework, some professors have a stricter definition of plagiarism than is strictly necessary. But, from what you say, you have cited the original and if it is clear enough that your citation implies that the source of the ideas is elsewhere, then it isn't technically plagiarism.
Note that whether you quote or paraphrase is irrelevant, assuming you quote properly (quote marks or a set aside of some sort). Plagiarism is about the idea itself, rather than the specific words.
Automatic plagiarism checkers do a poor job, actually, giving both false positives and false negatives. But if you cite (what the checker said overlapped) properly, it isn't plagiarism.
Another issue, frequently forgotten, is that for some sorts of things, especially in, say, math, there is effectively only one way to say something and some things are also common knowledge. So, for these things, neither plagiarism nor copyright actually apply. This probably doesn't apply in this specific case, but keep it in mind, generally.