Relationship problem, need some advice :( ?

ok me and my gf have been dating for 3 months, she has a son, and the dad is an active part of their life, they are no longer together, and she does NOT want to go back with him, we got in an argument last night, because she went for something to eat with her baby daddy, and i felt that i had to wait till she was done with him to hang out with her, she told me, she simply got something to eat, and got dropped off, and i trust her, it just weird to me, because at beginning, they argued, and she talked about him in a very bad light, and now there getting along, and she just says that she likes to get along, she told me last night, that wenever me and her argue, it makes her miss him more, what would i take from this, she got mad at me for explaining my feelings, that im being stupid,