Does inserting an IUD render a woman a Niddah even if there is no bleeding? If there is bleeding can it be attributed to a wound and thus the woman would not be a Niddah? (sources)
Noda Beyehuda (T. YD 120) and Aruch Hashulchan (YD 188) says she is Niddah because א"א לפתיחת הקבר בלא דם Chazon Ish says she is not Niddah since פתיחת הקבר בלא דם is not an issue when open externally. Minhag Yerushalim was to be Machmir. (שו"ת מלמד להועיל חלק ב (יורה דעה) סימן סה) Melamed Lehohil however is Meikel.
Reb Moshe Feinstein is Meikel when device is less than the thickens of small finger.שו"ת אגרות משה יורה דעה חלק א סימן פט
אם אינו עב מאצבע קטנה יש להקל Thus when inserting the IUD the woman is not Niddah even if פתיחת הקבר בלא דם is an issue, when open externally.
Yoatzot.org acknowledges there are differing opinions about this. Rabbi Yehuda Herzl Henkin's opinion is that insertion per se does not render a woman a Nidah; bleeding from removal is attributed to a wound; and bleeding from insertion will depend on the circumstances.
Regarding the second half of the question; whether bleeding after the initial insertion renders a woman a niddah, R. Eliyahu Abargel is lenient (to assume the blood is not menstrual) in a case where a woman has a vesset kavua; a set cycle, (the case he happens to be discussing), and the bleeding is not at the time of her vesset. (Mesorah L'Yosef (Qafih) Vol. 1-2; 2007, p. 16).
He cites R. Qafih (there) who rules unequivocally that such blood is not considered to be menstrual blood, and does not render a woman a nidda.
Regarding the first half of the question, whether the insertion renders her a nidda, this would be based on the Mishna Nidda (3:1) in which R. Yehuda is quoted as saying that anytime a woman miscarries, even a piece (of fetus) she is rendered a nidda, even if there is no accompanying blood. Rambam explains in his commentary to that Mishna (as per Nidda 21a) that R. Yehuda is of the opinion that anytime something leaves the uterus, bleeding inevitably follows.
However, Rambam rules (there and in Hilkhot Issurei Biah 5:13) in accordance with the Tanna Kamma (first opinion in the Mishna), that such a miscarriage does not render a woman a nidda, as we do not presume that she bled. Accordingly, in the case of the IUD there would be no reason that the insertion itself would render her a nidda.
Even according to the view of R. Yehuda which is adopted by other Rishonim, it is not at all clear that this would apply to something entering the uterus. I am not aware of any Rishonim who indicate that it would apply. Indeed, while the Noda Biyehuda (Tinyana YD 120) opines that it would apply, the Hazon Ish (YD 83:2) disagrees. For more views who dispute the Noda BiYehuda's view, see Hagahot Barukh Ta'am to the Noda BiYehuda and Shu"t Ateret Hakhamim (18) at length, who notes that Rambam, too, disagrees with the Noda BiYehuda (besides for the fact that as noted Rambam does not even hold of the opinion of R. Yehuda in the first place.