Was Wu Zetian the concubine of Li Shimin, and later became a queen?

Yes, Wu Zetian was originally Li Shimin's concubine.

In November of the 11th year of Zhenguan (637), after Emperor Taizong came to Luoyang Palace (Wei Zi City), he heard that 14-year-old Wu Zetian was beautiful and dignified, so he called her into the palace and named her "Wu Mei", which was later mistakenly called Wu Meiniang.

However, Wu Zetian didn't get Li Shimin's favor. She has been a talent for 12 years, and her status has never been improved. However, during Li Shimin's serious illness, Wu Zetian and Prince Li Zhi began to establish feelings.

In the 23rd year of Zhenguan (649), Li Shimin died, and Wu Zetian became a nun with some childless concubines in Ganye Temple in Chang 'an, but she has been alienated from the newly acceded to the throne, Li Zhi, Tang Gaozong.

Extended data:

After Li Shimin's death, Wu Zetian married his son Li Zhi:

As early as when Li Zhi was a prince, he had an affair with Taizong Wushi, a gifted scholar four years older than him. After the death of Emperor Taizong, Wu followed the example of the Tang harem and went to Ganye Temple to have a haircut for Nepal.

In May of the first year of Yonghui (650), I met Wu again when I was worshipping incense on the anniversary of Taizong's death. The two men recognized each other and told each other their thoughts after leaving.

In May of the 2nd year of Yonghui (65 1), Tang Gaozong's mourning was full, and Wu Shi entered the palace again. In May of the following year, he became a second-class Zhao Yi. Wu Zetian entered the palace again, knowing the queen's mind, so she obeyed the queen.

Queen Wang praised her constantly in front of Gaozong, which deepened Gaozong's love for her. Gaozong's love provided her with a ladder to climb step by step.

When Wu Zetian was in favor, both Queen Wang and Xiao Shufei were left out in the cold. Finally, Emperor Gaozong decided to abolish the king and establish a queen. He also knew that this matter was very serious, so he consulted the relevant ministers in advance.

In October of the sixth year of Yonghui (655), Emperor Gaozong no longer considered the opposition of Yuan Laopai, and wrote a letter to abolish the king. 1 1 After Wu Zetian was appointed, the queen officially changed hands.

Baidu Encyclopedia-Wu Zetian