Praise be to Allah.
It should be noted that tahaarah (purity) is not a condition of ihraam, and the menstruating woman may enter ihraam for ‘Umrah or Hajj and do all the things that other pilgrims do, except for circumambulating the Ka’bah (tawaaf). It was narrated that ‘Aa’ishah (may Allaah be pleased with her) said: “Asma’ bint ‘Umays gave birth to Muhammad ibn Abi Bakr in al- Shajarah, and the Messenger of Allaah (peace and blessings of Allaah be upon him) told Abu Bakr to tell her to do ghusl and enter ihraam.” Narrated by Muslim, 1209. Women who are menstruating or bleeding following childbirth come under the same rulings. And the Prophet (peace and blessings of Allaah be upon him) told ‘Aa’ishah, when she was menstruating, to do the same as other pilgrims did, apart from tawaaf around the Ka’bah. Narrated by al-Bukhaari, 1516.
If this woman did not enter ihraam for ‘umrah when she got her period, and she passed the meeqaat without intending to do ‘Umrah, then when she reached Jeddah she decided to do ‘Umrah, then there is nothing wrong with that, and she should enter ihraam from where she is in Jeddah, because the Prophet (peace and blessings of Allaah be upon him) said: “Whoever is closer than that should (enter ihraam) from where he forms the intention.” Narrated by al-Bukhaari, 1524; Muslim, 1181 – i.e., whoever is closer than the meeqaat should enter ihraam from where he is.
But if she intended to do ‘Umrah from the moment she passed or came in line with the meeqaat, but she did not enter ihraam from that point, then she has to go back and enter ihraam from the meeqaat. If she does not do that and she enters ihraam from Jeddah, then she has to offer a sacrifice and slaughter a sheep in Makkah, and distribute it to the poor and needy of the Haram.
Ibn Qudaamah (may Allaah have mercy on him) said: In conclusion, whoever passes the meeqaat without ihraam, intending to perform the rituals, has to go back to it and enter ihraam from there, if he can, whether he passed it knowingly or unknowingly, whether he knew that this is haraam or not. If he goes back to it and enters ihraam from there, then he does not have to do anything else, and we do not know of any differing opinion concerning that. This is the view of Jaabir ibn Zayd, al-Hasan, Sa’eed ibn Jubayr, al-Thawri, al-Shaafa’i and others, because he has entered ihraam from the meeqaat from which he was commanded to enter ihraam, so he does not have to do anything. But if he entered ihraam from a point closer to Makkah than the meeqaat, then he has to offer a sacrifice.
From al-Mughni, 3/115
And Allaah knows best.