Step daughter?

My boyfriend has a 4 year old daughter who we have Saturday and Monday nights. (She is a little delayed and has trouble getting her thoughts across). He picked her up when he was out of work and brought her back here. I was not there yet but he told me She cried hysterically for 40 minutes saying she wants mommy. He brought her back to her moms. About 2 hours later, her mom brought her back here and she cried again. At that time, I was nursing my son to sleep. I could hear my boyfriend getting upset and not being very comforting so I put my son to bed and went to see her. She was crying so much saying “mommy” and looking in every room for her. My heart broke I felt so bad :( I comforted her and she was okay after a few minutes. She doesn’t seem like herself though and it’s making me sad. She’s never done this before and she’s always very happy when she’s here. I have a 9 year old daughter whom she is best friends with. She also loves her little brother. I treat her like she’s my own baby I love her. We both treat her very well. I’m not sure why she was so upset. I was thinking she just really misses her mom? Idk :( Help please!! Thank you