What to do when u r confronted by your boyfriend's baby mama? they r not together, but didn't know we were?

I have been with him for a year, and they have a son together. He sees his son on weekends. Him and his bm been together for 7 years and he takes care of her and there child, even when she was bout to get kicked out her house he wanted to take her in, or give her money to pay rent. I know he cares about her a great deal but i am not sure if he's truly done with her. I know she wants to b back with him, i can tell. I also know that my bf loves me, i know i have his heart. Even though i have his heart he still doesn't want to tell his bm about me for fear she will sue him for child support and take there child away from him. I love him with all my heart and i don't wanna lose him, but how can i compete with their bond that they share. Anyway his bm cought us together at his house and she ask me if he was my bf i said no knowing for now it was up to him to tell her otherwise i dodn't want to mess things up for him. I left and she left too, luckly no drama, anyways i went back to check up on my bf and saw he was crying and he ran after his bm saying he was afraid she might do something crazy. As i was standing there watching him run after her, i asked myself y? Y would he go after her like that if he doesn't love her, and y leave me standing there? Am i crazy? should i b a little more understanding? or does he have unresolved feeling for her and that i should leave him alone. I love him so much and the thought of leaving him hurts like hell, please help.