I very much got the impression that it was Aurora she was in love with. She told Robin she had to go talk to someone "before it was too late", which would give the impression she was going to confess her love to the person she loved.
Then we see her with Aurora. She asks if Phillip is there and Aurora says no, Mulan looks relieved, then says "its you I came to talk to." After Aurora spills her news first, Mulan makes the snap decision to join Robin, then walks away in tears.
If I were in love with someone, and made the decision to tell them, my first stop wouldn't be to tell their S.O.
Yes, Mulan fought alongside Phillip, but she also spent a long time alone with Aurora after the encounter with the dementor guys.