If a menstruating woman puts her hand in water, does that make it najis?

The basic principle concerning water is that it is pure and a means of purification, and this description is not cancelled out unless some impurity falls into it that changes its colour, taste or smell; in that case the water is deemed to be najis (impure). 

The fact that a woman who was menstruating put her hand in the water does not affect the water or make it impure, because the body of a Muslim is pure (taahir) whether he is junub or whether a woman is menstruating, because of the report narrated by al-Bukhaari (276) and Muslim (556) from Abu Hurayrah, according to which he was seen by the Prophet (blessings and peace of Allah be upon him) on one of the streets of Madinah when he was junub. He slipped away and did ghusl, and the Prophet (blessings and peace of Allah be upon him) noticed that he was not there. When he came to him he said: Where did you go, O Abu Hurayrah? He said: O Messenger of Allah, you saw me when I was junub and I did not want to sit with you until I did ghusl. The Messenger of Allah (blessings and peace of Allah be upon him) said: “Subhaan Allah! The believer does not become najis.” 

An-Nawawi (may Allah have mercy on him) said: This hadeeth sets out a great principle, which is that the Muslim is pure in life and in death. In life he is pure (taahir) according to scholarly consensus…

End quote from Sharh Muslim 

He also said: The scholars said: It is not makrooh to lie down with a menstruating woman or kiss her… And it is not makrooh for her to put her hand in any liquid … Her cooking, dough and other things she makes are not makrooh; her leftover water and sweat are taahir (pure). There is scholarly consensus on all of that. Imam Abu Ja‘far Muhammad ibn Jareer narrated in his book on the madhhabs of the scholars that the Muslims are unanimously agreed on all of that, and the evidence for it in the Sunnah is clear and well known.

End quote from Sharh Muslim 

Al-Khuraqi (may Allah have mercy on him) said: If a menstruating woman, one who is junub and a mushrik dip their hands in water, it is still taahir. 

Ibn Qudaamah (may Allah have mercy on him) said: With regard to the purity of water, there is no confusion concerning that unless there is anything impure on their hands, because their bodies are pure and what they are do not imply that their bodies are najis. Ibn al-Mundhir said: The majority of scholars are agreed that the sweat of one who is junub is taahir. That was proven from Ibn ‘Umar, Ibn ‘Abbaas and ‘Aa’ishah (may Allah be pleased with them) and from other fuqaha’. ‘Aa’ishah said: The sweat of the menstruating woman is taahir. All of that is the opinion of Maalik, ash-Shaafa‘i and ashaab ar-ra’y, and no difference of opinion has been narrated from anyone else.

End quote from al-Mughni, 1/135 

Based on that, your ghusl is valid and there is no need to repeat it. 

And Allah knows best.