Relationship problem

My girlfriend tells me that she’s going to a friends house. She goes home and she calls me and told me that she went to her ex’s house and told me that her ex kissed her. At that point she lied to me where she was going. After she told me that she said that she went to her ex’s house because she wanted to be friends with her. But I feel like she knew her ex wanted her back and she went over knowing that her ex still had feelings for her and stuff. I don’t know what to think. Is this considered cheating? Is it a bad thing?