New International Version
But she was unfaithful [H2181] to him. She left him and went back to her parents' home in Bethlehem, Judah.
The Hebrew word here has a wide range of meanings.
1 be or act as a harlot ...
2 figurative of improper intercourse with foreign nations ...
3 of intercourse with other deities, considered as harlotry, sometimes involving actual prostitution ...
4 moral defection
Here is some background info about adultery and prostitution.
It was serious, worthy of death as described in Deuteronomy 22:22
If a man is found sleeping with another man’s wife, both the man who slept with her and the woman must die. You must purge the evil from Israel.
Even if the husband was just suspicious, it was serious. He would bring her to the priest according to Numbers 5
19 Then the priest shall put the woman under oath and say to her, “If no other man has had sexual relations with you and you have not gone astray and become impure while married to your husband, may this bitter water that brings a curse not harm you. 20But if you have gone astray while married to your husband and you have made yourself impure by having sexual relations with a man other than your husband”— 21here the priest is to put the woman under this curse—“may the Lord cause you to become a curse among your people when he makes your womb miscarry and your abdomen swell. 22May this water that brings a curse enter your body so that your abdomen swells or your womb miscarries.”
“ ‘Then the woman is to say, “Amen. So be it.”
The husband didn't do any of these damaging things to her. Instead, he did the opposite. He forgave whatever that she did.
Let's see the context:
Judges 19:1 In those days Israel had no king.
Now a Levite who lived in a remote area in the hill country of Ephraim took a concubine from Bethlehem in Judah. 2 But she was unfaithful to him. She left him and went back to her parents’ home in Bethlehem, Judah. After she had been there four months, 3her husband went to her to persuade her to return. He had with him his servant and two donkeys. She took him into her parents’ home, and when her father saw him, he gladly welcomed him.
She initiated the leaving. To where? To her father's home. If she had committed prostitution, it's unlikely that her father would welcome her back.
When her father saw the son-in-law, he was glad. Why? If she had committed adultery, both the father and the concubine were in big trouble. It didn't seem that she had committed anything that serious. The three seem to be getting along well. The father kept wanting the husband to enjoy his hospitality.
Finally, the husband traveled a long way to get the concubine back. After she died a terrible death. He wanted justice. Would he have done it and raised such an outcry, if she was a prostitute? I don't think so.
Cambridge Bible suggests that it could be as simple as that she was just angry with her husband without being unfaithful:
played the harlot against him] The text is open to suspicion. LXX. cod. A reads was angry with him; this suits the context, which implies a quarrel, but not unfaithfulness, on the woman’s part; she left him in anger and returned to her father’s house, whither the Levite followed to pacify her (Jdg 19:3 f.). How are we to account for the reading of the text? Moore ingeniously suggests that by the transposition of two letters she was angry (te’ĕnaph) might have become ‘she committed adultery’ (tin’aph), which was altered by the Jews to ‘played the harlot,’
I don't know what she did wrong that caused her to run away from the husband. Whatever it was, I don't think it was prostitution or adultery.